Abraxas, the antithesis of Eternity, exists only to destroy anything and everything in all creation. Keeping it imprisoned is one of the reasons Galactus needs the life energy from devoured worlds. The one time Abraxas did break free, the results... weren’t pretty.
King Thor: The resurrected Gorr decides that rather than just killing all the gods, he's going to wipe out all life by destroying the universe itself and start it over without gods.
Man-Thing: Amodeus Q. Termineus sought the shattered fragments of the Nexus of all Realities. With them in possession he could destroy the Multiverse.
Annihilus became one of these during the Annihilation. His vast fleets carved a path of devastation across the galaxy, used a large, tick-like weapon called the Harvester of Sorrows to drain entire planets of their energy to feed on, nearly wiped out the Kree and Skrull empires, and (with the help of Thanos and two cosmic superbeings) imprisoned Galactus himself. Why is he an Omnicidal Maniac instead of just another alien conqueror? Annihilus' true motivation for capturing Galactus was to turn him into a bomb. A bomb that would kill everything in the universe except for himself. What's truly scary is that he came damn close to pulling it off, too.
Amatsu-Mikaboshi from the Chaos WarCrisis Crossover is also this. He existed long before the universe did and, in the Chaos War, wants to return things to that state. Marvel sure is in love with this trope.
The Cult of Entropy seeks to speed up the destruction of the Marvel Universe/Multiverse.
Doctor Octopus seemingly became this during the "Ends of the Earth" storyline. Knowing he was dying, his plan was to use powerful satellite weapons to kill 99% percent of the population of the Earth so that the remaining 1% and all of their descendents would remember him forever. He nearly succeeded. However, just as he was about to pull the switch, Spider-Man pointed out a flaw in his plan: the incredible heat caused by the weapons would leave all the survivors brain-dead. This revelation caused Octopus to hesitate long enough for the hero to pummel him into unconsciousness. However, it is then revealed that Octopus pulled a Mind Swap on Spider-Man and was then running around as Peter Parker, vowing to do a better job being Spider-Man than he ever did. It is ambiguous if Octopus thus had a change of plans or if the attempt to wipe out most of mankind (along with the flaw therein said attempt) was just an elaborate ruse to pull of the Mind Swap plan all along.
Guardians of the Galaxy: The Badoon are entire species of this (or at least the males are. The women of the Badoon are actually quite pleasant). Several times, it's been made clear if given half the chance, they absolutely would kill everyone and everything in the universe that's not a Badoon. In at least one reality, they have.
All of the Elders of the Universe of Marvel Comics came up with a purpose to keep from going mad from their immortality, but one of them, the Obliterator, was clearly insane from the start. After murdering his entire species, he wandered the universe looking for things to kill, using his powerful high-tech weapons to shoot at anything that moved. (Except the other Elders, the only beings who come close to being his allies.) Fortunately for the universe at large, he has no real confirmed super-powers other than the immortality that all the Elders have (possibly making him the least powerful of the group) and is also not very smart.
Immortal Hulk: The One Below All, the Evil Counterpart of the One Above All, is so disgusted by the very existence of the multiverse it wants to destroy everything, leaving only a dead and broken multiverse, just so it can be alone.
Maelstrom is the champion of Oblivion and constantly schemes to destroy everything.
On a more planetary scale, Onslaught started out as a mutant supremacist of the same stripe as his psychic 'father', Magneto. After seeing the Age of Apocalypse in Bishop's memories, in which mutants really did rule the world and destroyed it, he decided no one was worthy.
Thanos is the most famous Marvel Universe example. He has a vision of Death as a beautiful woman and in order to win her favor he embarks on a quest to wreak death and destruction. Although he has stated on multiple occasions that the elimination of all life might destroy Death as well as it would eliminate her need to exist, so he would eliminate half of all life, thus ensuring Death still has a steady supply of souls.
In The Thanos Imperative Thanos goes completely insane when he realizes that his Thanatos Gambit to destroy the Cancer Verse also rendered him immortal, meaning he can never be with Death. He makes an earnest effort to wipe out all life in the Marvel-616 Verse and has to be sealed in what's left of the Cancer Verse to stop him from pulling it off.
Ultimate X Men: Most plans of Magneto aim to a worldwide culling of the human race. And he finally achieved that during Ultimatum.
X-Men villain Cassandra Nova takes a strange approach to this. Due to the circumstances behind her existence (namely, as the self-constructed remnants of an Evil Twin killed in the womb), she's convinced she and Charles Xavier are both still in the womb, and the only real things in existence; everything else is but an ideation of theirs, a piece of their imagination. And since she's so fundamentally opposed to Charles, she wishes to destroy this "imagined" universe completely just to torment him, starting by those mutants he champions so much. On the latter count, she's sixteen million corpses in and counting.
X-Men '92: Original Generation villain Xodus the Forgotten combines this trope with Final Solution, as he is a Fantastic Racist Celestial who traveled from world to world exterminating any and all Mutant populations until his Celestial brethren made him a Sealed Evil in a Can. Predictably, the seal is broken, and he heads to Earth to pick right back up where he left off.
In Doctor Doom, Doctor Doom becomes this in the final issue when he destroys an entire universe because he hates what his counterpart (whom he had just murdered) did to make it a paradise. Namely, the alternate Victor became a humble man who befriended Reed Richards and worked together with him to make the universe a better place. Doom would rather kill a universe than accept that he should let go of his pride and his hatred of Richards.
In X-Man (Counter-X), Qabiri of the Brilliant City takes this path. Most travelers between alternate worlds move 'sideways', but Qabiri's people see the multiverse as a spiral where not all worlds are equal. And as other, lesser, races from flawed worlds start to realize that some of them will start climbing the spiral and polluting the perfection of the Brilliant City with their presence. Thus, Qabiri has chosen to save his home by leaping into the darkness of those broken universes and killing their Earths before they reach his.
Talita from Terror, Inc. (MAX) was trapped between life and death in a void of blackness for 800 years. She went completely mad and came to love the Void. After she was resurrected, she feared her commitment to the Void: "I feel... a strength coursing through me... yes... yes... I worried I might lose faith as the pain left me. I feel joy, strength, lust all raging through me... ALL PALE BEFORE THE EMPTINESS". Before she ends the world, she wants to taste life followed by nothing.
Marvel Two-in-One Annual #5 had Pluto attempt to use a device to overload a black hole in the hopes of destroying the universe so he could rule over eternal darkness. The Hulk and the Thing foiled this centuries-long plan by smashing the ground underneath him really hard so he fell into his machine.
