The evil and non-evil versions of insanity pop up in a lot of the Marvel Universe's more powerful human characters, apparently as a way of Holding Back the Phlebotinum. Scarlet Witch, Phoenix, The Sentry, or anyone else like that being able to use their maximum power levels while fully themselves would remove all drama. Scarlet Witch is currently the most powerful and resultantly has it the worst. We haven't seen her mind in stable condition for quite some time now that her power's gone from "I point and my opponent slips on a banana peel" to "whatever I decide simply is."
The Serpent Crown is a recurring artifact in the Marvel Universe that grants great power, however it will corrupt you as you will be mentally dominated by Set.
When the Canadian government was looking for people to join Alpha Flight, they initially had trouble finding recruits. The people in charge of the program decided to try creating their own superbeings, and they got the bright idea to experiment on a Serial Killer who got a pardon in exchange for agreeing to participate. The result was a crazed monster with deadly psychic abilities calling itself Bedlam. Wolverine had initially signed on to join Alpha Flight, but this debacle was what caused him to leave the group in disgust. Fortunately, Guardian left an emergency protocol that would automatically call Wolverine should Bedlam be freed, which proved necessary when a government official unfamiliar with the backstory ended up setting him free.
Madison Jeffries, a Technopath, and his brother Lionel, a doctor with Healing Hands, fought in a war together, where Madison could forget about his mutant powers, while Lionel used his to heal wounded soldiers. Then one day, some soldiers were brought back in pieces. Lionel attempted to put them back together, and when he couldn't bring them Back from the Dead, he went insane and started using his powers to mutilate everybody within arm's reach, becoming Scramble the Mixed-Up Man. Madison had to use his powers to create a containment suit for his brother, and had him locked away, completely isolated from human contact. Scramble was eventually able to seemingly fix himself by using his powers on his own brain, but it was a temporary fix at best, and his lack of ethics helped to push him around the bend again, forcing Madison (now using the Box robot) to put him down for good.
All attempts to replicate the Super Serum that gave Captain America his powers have either made people go crazy or been used on someone who was already crazy. In fact, in the Ultimate universe, this seems to be the origin for all of Ultimate Spider-Man'sRogues Gallery. One of the rare good endings for someone getting the Super-Soldier serum is Isaiah Bradley, grandfather of Patriot from the Young Avengers. While he was stable, he would end up gradually suffer brain damage over time despite not aging much physically. Yes, that's what passes for a good ending when trying to reproduce the Super-Soldier serum (though granted, Isaiah's condition was exacerbated by the horrific conditions he was kept in after his mission.) One wonders why they keep trying.
Deadpool was probably messed up before developing terminal cancer, but the Weapon X program (which initially failed to give him a Healing Factor) gives him a hard shove in that direction. Then Dr. Killebrew experiments on and tortures him to the point of having visions of (and falling in love with) Death. What finally demolishes his sanity is when Killebrew orders him killed, his healing factor finally kicks in, saving his life, making his disfigurement permanent, and causing Death to reject him. Depending on the writer, he's a mix of Ax-Crazy, Deadpan Snarker, gleeful Genre Savvy, and Medium Awareness. Another issue is that the Healing Factor is connected to his cancer — he's basically an immortal living cancer. The constant state of flux and strain this places on him (including his brain) is why he's so unstable.
The Incredible Hulk. The gamma bomb gave Bruce Banner huge power and exacerbated his multiple personality syndrome. Plus, the madder he gets, the stronger he becomes.
Interestingly, for both The Mask and Marvel's gamma ray mutants, what happens to the subject's mind depends on what part of their personality they had dissociated themselves from. Most people who get the Mask unlock their evil side, but the fellow in The Movie didn't have an evil side, only a chaotic side, so he essentially became a Looney Tunes character. Marvel goes into more detail — Banner suppressed the rage that came from being abused as a child, She-Hulk suppressed her sexuality, Doc Samson suppressed his desire to be a hero, and the Abomination suppressed his self-hatred.
Handled interestingly with Sentry. His powers are like some ridiculous combination of Superman and Franklin Richards, making him technically unstoppable and all-powerful. He's also a paranoid schizophrenic who managed to convince himself that there was an evil galactic power called The Void that would destroy the earth if he stayed a superhero... and then actually created it out of thin air, making a problem for The Avengers to handle while Emma Frost gave him some emergency psychotherapy. During this time, he also managed to Retcon himself out of his own universe, so that his Golden Age exploits all became some comic writer's fantasy. In something of a subversion, Sentry's not a villain: in current canon, he works to use his powers for a great deal of good. Unfortunately, his psychosis still isn't fully under control, and it's a disability that is sometimes just impossible to work around.
Later developments suggest that the Sentry is an inversion of this trope. Robert Reynolds was already a mentally unstable drug addict before taking the serum that gave him his powers, so it's more like someone with great insanity given great power. Furthermore, Reynolds didn't become the Sentry and create the Void, it was the other way around...
An alternate interpretation is that Reynolds splits into the Sentry and the Void, neither of which are more physically or psychologically real than the other.
In Uncanny Avengers, he returned as the Horseman of Death (the Void apparently got bored and wandered off), even crazier than ever. Eventually, Doctor Strange managed to cure him, and by their mutual agreement, created a pocket dimension inside his head where he could live so his body could float in deep space and be no trouble to anyone. Unfortunately, Loki tricking his way into becoming Sorcerer Supreme meant that Strange hit his Godzilla Threshold and called on the Sentry's help... which, while in a superficially indirect fashion, accidentally unleashed the Void. The Sentry was not pleased.
Most recently, he ended up merging his nature as Robert Reynolds with the Sentry and Void personas, becoming even more powerful than ever, with ominous black hair and wearing an unnerving red and black costume. However, he's not strictly evil, more like a fairly hardcore Anti-Hero.
In Spellbound (1988), the main conceit of the series is that using the Spellbinders' power will drive the wielder mad no matter what, and it's only a question of when it will happen to Erica.
Spidey's Arch-Enemy, the Green Goblin, in his original portrayal, was this. He was a distant father whose business practices were not always scrupulous, but he had redeeming traits, such as genuinely loving his son, and saving Gwen Stacy's father's life. Then the formula that gave him his powers drove him insane, though periodic bouts of amnesia restored him to his former self. After he killed Gwen Stacy, however, he was rewritten as always being sociopathic, with the kinder personality that he possessed during his periods of amnesia being a false personality. The Goblin formula probably enhanced his insanity, however.
Spider-Man Unlimited (2023) reveals that the Spot suffers from this. The interdimensional portals that constantly shift along his body sometimes end up reaching his brain, and as a result parts of it keep getting flung off into other dimensions, affecting his intelligence and ability to focus. This was used to explain the character's shifts from two-bit bank robber to genuine threat. Notably, Spot with no portals on the brain is just as criminal and villainous as he is when he's impaired: he's just more competent.
Apparently, Chris Claremont likes this one, or used to. The Phoenix being a cosmic entity was a Retcon to satisfy the then-editor-in-chief's requirement for bringing Jean Grey back: she had to be innocent of her crimes as Phoenix. (The destroyer of five billion lives couldn't very well be welcomed back to the team with open arms. In fact, her original Heroic Sacrifice was mandated for that very reason.) The original story portrayed Jean's cosmic powers as the ultimate expression of her abilities, and the change from hero to Anti-Hero to cosmic-scale threat as simply the result of having the sort of powers she now possessed. Storm also began a similar change upon maxing out her powers, but thankfully was able to return to her previous self (her power level returning to normal with it) within that issue and before she did anything particularly heinous.
The 'return' of Jean Grey in the X-Factor retread of the original X-Men was so badly done that it left permanent damage to the storylines of the Marvel Universe. The obvious moral cop-out of 'it wasn't really her' not only undid the basic point of one of the landmark storylines of the MU, but was done in a half-assed way, because they tried to claim that Jean deserved the credit for the self-sacrifice of the Phoenix entity, but not the blame for its crimes, even though both supposedly derive from the human element from Jean. Sorry, folks, you can't have that both ways.
To make it even worse, Claremont clearly hated the idea that the Phoenix entity was not Jean, he kept trying to sneakily re-retcon it back, and the story got into the hands of other writers and mutated further, eventually becoming a total, unworkable, self-contradictory Continuity Snarl. Nowadays, it's mostly ignored.
Oddly enough, though, Jean's time traveling daughterRachel, who already had ample reason to have gone insane (but didn't) before acquiring the Phoenix power, managed to wield it for years without going crazy. And then lost the power (despite the Phoenix itself insisting that it had permanently merged with her).
Long story short, Jean's power level, mental state, and the effect these have on each other all depends on where Jean ends and the Phoenix begins. Too bad no two mentions of the Phoenix in a row give the same answer on that score. However, the cosmic critter isn't malevolent. Mastermind screwing with her head was what turned Phoenix into Dark Phoenix, and to suggest it might happen again much later in Phoenix: Endsong took a botched resurrection to again put the Phoenix out of whack. Poor Jean will Never Live It Down, despite this.
Oddly enough, Jean and her relatives actually tend to handle vast/cosmic scale power pretty well, Phoenix saga aside. Jean spent most of the 90's and early 2000's being incredibly powerful under her own steam, and her teenage counterpart has largely kept her powers in hand too. Meanwhile, her children, Rachel, Nate, and Cable, are all vastly powerful (intermittently, in Cable's case, and sometimes Nate's) and entirely sane. Usually. Cable seemed to undergo this when his Techno-Organic virus went into remission and he created the floating island of Providence, jacking him up to the point where he could fight the Silver Surfer on even footing, hold Providence in the air, and repair the landscape around them as they fought. As it turned out, however, it was a Genghis Gambit to get the world to unite against him. Nate, meanwhile, lived in mortal terror of a combination of this and lethal cosmic scale Power Incontinence, but largely held it together pretty well - which, for a Living Weapon who had no functional childhood, grew up in the hell that was the Age of Apocalypse, had to live with the knowledge that he was slowly dying, and had just about everyone trying to manipulate or destroy him once he got to 616, is really quite impressive. As for Rachel, of the lot of them, she's by far the sanest.
Then in Uncanny X-Men (2018), Nate appeared to a) regain his powers and then some, b) lose his marbles and declare his status as a god/messiah. Considering his status as a Messianic Archetype and raw power, it's a bit hard to argue with. However, it's revealed at the end of the arc that he got his powers back, but at the price that he's dying, again, and he's desperate to do something good with his powers while he can.
He then created the Age of X-Man, a plane of existence meant to be directly opposed to the dystopia he grew up in, an attempt at utopia (specifically, he wanted to help the X-Men by breaking them out of their constant cycle of conflict and grief). Unfortunately, through a combination of his Control Freak tendencies, attempts to disassociate himself from humanity, and screwed up background, it instead drifted far too close to a creepily cheerfulNineteen Eighty-Four clone. At the end of the arc, he conceded his mistakes and realised that he was Not So Above It All (though the comic noted that Both Sides Have a Point) and let the X-Men go, before rewriting the reality with the assistance of AOX!Magneto to improve it and remove the Orwellian aspects.
While Stryfe and Maddie Pryor are worth noting, both were clones - Stryfe remains in denial about this, while Maddie did not take it well - and Stryfe was raised by Apocalypse, while Maddie was created by Sinister as his weapon with cobbled together memories from Jean Grey. Their insanity was not exactly of their making.
This trope as related to the Summers family is further deconstructed in the character of Tyler Dayspring, son of either Cable or Stryfe (raised by Cable, but it's heavily implied that Stryfe is his true father). Like Stryfe and Maddie above, Tyler lost his sanity through means not of his own making (he was brainwashed in the future by one of Stryfe's minions) — but unlike them, he has very weak mutant powers, to the point of needing to augment himself with Apocalypse's technology just to stand a chance against the heroes. In a way, Tyler got the rawest deal out of the entire Summers line — all the insanity of this trope without any of the power.
Some writers have tried to pull the same thing with Havok (Cyclops' brother), who controls (or at least, has a degree of access to) the Power Cosmic, another fundamental force of the Marvel universe. This is made apparent with the introduction of the long-anticipated third Summers brother Vulcan, who has near-Phoenix level command of the Power Cosmic (and would be near-Phoenix level dangerous if he had more than two brain cells to rub together) but at the cost of being completely off his rocker.
Magneto comes close to saying this trope by name in issue two of the nineties X-Men series. When Moira MacTaggert explains how his powers played havoc with his mind, he states: "What, with great power comes mental instability?"
Magneto's daughters, Polaris and the Scarlet Witch, suffer from similar sanity-damaging "cursed" powers. Insanity might be In the Blood where this family's concerned.
It's implied that the Scarlet Witch's crazy came from the Mind Screw the Avengers did so she'd forget her maybe/sort-of/magic children rather than from her powers themselves. Similarly, Polaris is only crazy when outside influence is involved... it just happens to her more often than it does to anyone else.
While not part of Magneto's family per se, a special mention should be made for his Dragon and intended successor from the 90s, Exodus. He has psychic Combo Platter Powers ramped up to Superpower Lottery levels, but unlike most mutants, who manifest by puberty, his powers didn't manifest until adulthood, and then not until being forcibly awakened by Apocalypse. The result is a character who projects constant wide-band Psychic Static where he goes, was implied to always be speaking telepathically and physically simultaneously, and at one point was even implied to suffer from a psychic variant of schizophrenia because his telepathy was so strong. He's developed greater control of it over the years, but even as recently as 2012 it was stated that he had never actually pushed the upper limits of his powers.
In the Age of Apocalypse this trope is inverted, as the Exodus there is shown as wearing Power Limiter devices and not being aware of a good chunk of what he can do until a Godzilla Threshold forces Magneto to reveal some of it to him. When questioned by Bishop, Mags stated outright he limits Exodus's knowledge of his powers to keep him from being corrupted by them.
This was lampshaded in the Assault on Weapon Plus story arc, where the Weapon Plus files stated that super soldier experiments on criminals and psychopaths yielded less than reliable results, prompting them to find a different method of creating anti-mutant super soldiers.
In newuniversal, which is also by Warren Ellis, John Tensen gains telepathic powers that let him "see" a person's misdeeds or ignoble intentions. The first time he used these abilities, he discovered that his own nurse was planning to poison him. Tensen, not surprisingly, became Ax-Crazy and is now the "worst serial killer in New York City history," to quote a minor character. It doesn't help, though, that Tensen had been shot in the head before he got his powers, and as a result starts to think he's in Hell.
Nova: The Nova Force is dangerous in large amounts. At one point Garthan Saal takes on the entirety of the Nova Force, and goes completely bug-nuts. It's because of this that several years later, during Annihilation Richard Rider is extremely reluctant to do the same. The Xandarian Worldmind has to assure Rich it'll be helping to prevent that happening, and aside from a brief, trauma-induced rampage, Rich remains mercifully un-insane. But as it transpires, the strain drives the Worldmind mad. When asked, a saner part of it just shrugs and figures Rich is too human to go nuts.
For a long time, it was explained that original Ant-Man Hank Pym's various mental issues were the result of the Pym Particles he used for his powers in the first place affecting his brain chemistry. While it was carried over to the Marvel Cinematic Universe, this is no longer the case in the comics, as it was later retconned (possibly due to the Wasp, Scott Lang, and Bill Foster staying relatively sane) that even before becoming Ant-Man, Hank was bipolar — and this wasn't the only retcon as the much-reviled The Crossing (the story that tried to say Iron Man was a Manchurian Agent for Kang) earlier attempted explain Hank's issues by saying that Hank was really Kang's first choice before he moved onto Tony and the various issues Hank suffered were really the result of this brainwashing not taking hold. However, much like retconning that it was really Immortus who enthralled Tony since only the events of Operation: Galactic Storm, Avengers Forever revealed that Immortus lied and really did nothing to Hank's mental state.
Captain Britain: Mad Jim Jaspers is an Omega-level mutant with Godlike reality warping powers that came packaged with the side-effect of driving him increasingly insane.
