Anyone revived via a Lazarus Pit suffers temporary insanity as a side-effect of the rejuvenation process. This becomes part of what turns Jason Todd into the Red Hood after he's revived by Ra's in an attempt to make amends for playing a part in Joker killing him.
An interesting case concerning pre-FlashpointAzrael. When Jean-Paul Valley was born, he was implanted with a brainwashing trance of sorts known as The System, which would grant him amazing strength and agility when he donned the gear of Azrael, though it made him Brainwashed and Crazy, pushing him to want to kill the guilty. When he first abandoned that role, he functioned pretty well... until the Scarecrow doused him with Fear Gas when JP took up the Mantle of the Bat in Bruce Wayne's place. The entirety of the subsequent Knightquest storyline has Jean-Paul constantly fighting The System until he lets Abattoir die. When he does, he finally submits to The System and becomes a fearful mixture of Batman and Azrael and forces Bruce to take back the Mantle.
Happens to Batman when he gets Superman's power during a battle with the Silver Banshee in a Superman/Batman story. Batman uses his new powers to bring complete fear and order to Gotham's criminal underworld, and eventually sets his sights on the world, but he becomes increasingly aggressive. With the aid of the Justice League, Superman is able to bring himself and The Dark Knight back to normal. Admittedly this was a Be Careful What You Wish ForArtifact of Doom.
Interesting case with Black Adam. His powers don't drive him crazy (he went crazy on his own, and sometimes comes back from that insanity...partially), but he can share them, and anyone who would take them on immediately turns evil. Anyone, including the goddess of love.
Doom Patrol: Mento wasn't the most stable/mentally healthy guy to begin with. Then he starts tinkering with a helmet that cranks his Psychic Powers up and takes up superheroing. But his wife and her team die, and it REALLY sent him downhill until he's doing stints as a supervillain and trying to kill his adopted son. When that heroing team was led by a fellow retconned into a Mad Scientist and Magnificent Bastard, it was damn near inevitable.
Firestorm (DC Comics): The curse of the werehyena causes anyone suffering it to gradually go insane. It is theorised that the madness suffered by the werehyenas is one's bestial side taking over, coupled with an exaggeration of negative emotions.
The Flash: Hunter Zolomon spent his life in a Trauma Conga Line, but he remained a good cop until Gorilla Grodd crippled him and the Flash, whom Hunter considered a friend, refused to use time travel to undo the damage. Hunter attempted to use the Flash's cosmic treadmill himself, but it blew up in his face. The resulting super powers had the side effect of scrambling his thought processes, until he decided that the best way to help his friend the Flash was to make him a better hero through tragedy as Zoom. Hunter still thinks he's helping the Flash, although he gets occasional flashes of My God, What Have I Done? when Wally foils his plans.
Green Lantern: Within the Emotional Electromagnetic Spectrum, Red and Violet are the colors that are the furthest from the center, and thus most prone to influence negatively on the ring's wielder. Red is fueled by Rage, thus making it susceptible to Unstoppable Rage, while Violet can easily fall into Love Makes You Evil and Love Makes You Crazy.
Retconned for Doctor Magnus, the leader of the Metal Men. He needs a careful application of medicine in order to stay stable and good. He's kidnapped along with genuine mad scientists and they cancel his meds, intending for him to regress to his previous level of insane creativity, in which he created a horrific weapon of mass destruction, the Plutonium Man. Though he does recreate the Plutonium Man, he destabilizes very quickly, and with the help of several sentient mini-Metal Men he'd managed to cook up in his lab, invents a gun with living ammo and goes on a rampage (against evil men only), screaming about how he really needed his meds.
The Psycho-Pirate, a villain with the emotion-changing Medusa Mask, actually had three of them, one for a single emotion, before deciding to combine all three to control all emotions. Unfortunately, continued use ended up driving him insane — he's first seen in Crisis on Infinite Earths locked away in an insane asylum.
Inverted in All-Star Superman: Lex Luthor temporarily gains Superman's powers, and while he predictably rampages, he finds himself stopping to examine the amazing perspective his newfound powers and super-senses give him. He eventually concludes that having the level of power and insight that Superman does would make people care for their fellow human beings, and mellows out considerably after losing the powers.
Superman: Up, Up and Away!: One Kryptonite Man is a scientist who thinks Kryptonite can be used as a safe energy source. When he himself becomes that energy source, he decides to show the world how effective it can be by... a murderous rampage. Later, another scientist goes cuckoo bananas when he gains control over an oversized amoeba. Or so it seems. Superman subdues the guy, who says he didn't want to do it, but Intergang, a powerful criminal organization, made him.
In A Mind-Switch in Time, Euphor becomes more powerful and more insane as he mass-absorbs negative emotions until he has become a crazy overlord who can give Superman a hard time.
Livewire, experiencing some Adaptational Heroism after immigrating to the comics, has her status as a Psycho Electro justified by the observation that, as an Energy Being, taking on more energy affects her equivalent of brain chemistry. In her case more power is literally more insanity. Putting her in Superman's suit from his blue period to regulate her form snaps her to sanity immediately and she becomes a hero at the end of Superman: Grounded.
Matthew Cable from Swamp Thing. His Psychic Powers and his spiraling alcoholic insanity both stem from the same illicit electroshock treatments.
Invoked but eventually subverted in the 1990s "Postboot" of Legion of Super-Heroes. Livewire is often spoken of not using his lightning powers to their utmost. He also keeps warning sister Sparks of misusing their abilties. It turns out Livewire is convinced that the lightning drove their older brother Mekt insane and into the criminal Lightning Lord. Even when Mekt is killing cops and attacking, Livewire begs him to stop so they can get him help. Sparks is amazed as she says that Garth looked up to Mekt too much as he was always this corrupt and twisted figure. After Mekt blows of his arm, Garth finally accepts the lightning did nothing but make an already psychopath Mekt worse to defeat him. This allows Garth to overcome his own problems and be able to use the full potential of his power.
This is an issue for gods, and the Greek pantheon actually split themselves in two in antiquity to create the Roman pantheon because the influx of worshipers was too much for them. In modern times they don't worry about it as much since they have far fewer worshipers and they've actually rejoined with their Roman counterparts. Some of the gods have trouble with it anyway, like Ares, Aphrodite and Dionysus because their power doesn't just come from worshipers and their attributes influence their mindset and actions.
Wonder Woman (1942): Hypnota goes off the deep end at the same time as they gain their powers, though if this was a side effect of their new mild telepathy or due to the damage left behind by the bullet through their head which activated their powers is not clarified.
Wonder Woman (1987): Once the White Magician's Black Magic use starts affecting him physically his mind goes right out the window and he becomes a twisted demonic thing before his death.
Wonder Woman: Odyssey: With every unjust death, Nemesis' power grew until she could reign alone above all other Greek dieties, whom all fled to the mortal plan in terror and went into hiding to escape her wrath. It also drove her completely mad.
In the New 52 arc Darkseid War, members of the Justice League end up becoming the New Gods after the death of Darkseid and others caused a power vacuum. Batman becomes the New God of Knowledge, taking Metron's chair for his own, and intends to his all-powerful knowledge to bring order to Gotham, even if preventatively. Superman becomes the New God Of Strength when Luthor throws him into a Fire Pit in a deeper attempt to recharge Superman's solar cells. It Goes Horribly Right since while it does make him much more powerful, Superman's temper is temporarily out of control (because, well, it is fire from Apokolips). The Flash becomes the new Black Racer after he kills the old one (namely, he was trying to convince Barry to take his place and required him to offer one life. When Barry initially refuses, the Black Racer threatens to give it to Reverse Flash or Grodd, thus Barry uses Exact Words and kills the Black Racer... before offering himself as said life.) Subverted with Hal Jordan. He becomes the New God Of Light at behest of the Mother Lantern (the Mother Box fusing with the Green Lantern core, but lacking a vessel for will to use). Hal brings back everyone killed during Apokalips' invasion of Oa, but he gives up godhood when he realizes it would mean giving up will.
