The Punisher has the goal of punishing the guilty at any cost, and while in most depictions of him have him respect a code of sorts (don't hurt innocents or the police, etc.), some writers have made him truly unfettered. In The Punisher: The End oneshot from the MAX imprint, he destroys humanity's last chance for survival in a nuclear wasteland rather than show mercy to the men and women who made the world into that wasteland. Even by Unfettered standards, that's chilling. And at the same time, it has a nihilistic nobility to it; Castle let humanity die out rather than leave it under the control of the people who brought the world to an end. Humanity's epitaph, as written by Frank Castle; "We died at the hands of evil men... but we did not let it go unpunished."
Abigail Brand, leader of SWORD, the space-focused equivalent of S.H.I.E.L.D.. When an ambassador from another planet told her that his world's prophets predicted a mutant would destroy their planet, she let him bring Colossus Back from the Dead and keep him as a guinea pig for a mutant cure rather than risk an interplanetary war. This kind of attitude does not endear her to the X-Men, but they end up working together anyway.
Thrown in her face in the end, as the prophets were in the service of a Woobie, Destroyer of Worlds who had gone insane from her civilization's obsession with domination through violence,("A world where murder is honor. Where children are nursed on the blood of the weak.") and decided to engineer the Mercy Killling of her entire world. She's notably humbled by the revelation that her lack of scruples enabled her to be manipulated to such an extent, which leads her to adopt the X-Men (Beast in particular) as Morality Pets.
Thanos just wants to be loved by Mistress Death and if he has to destroy the whole universe, so be it.
The Ultimate Marvel version of the Hulk. In every way that Banner represses and limits himself; emotionally, sexually, socially, the Hulk has a complete lack of inhibition or limits. His only goal? Torment Banner. Banner refuses to eat meat. Hulk eats people. Banner doesn't act on his attraction to Betty Ross. Hulk keeps a harem of concubines.
In the mainline Marvel Universe, we have the Grey Hulk, an amoral hedonist who had no qualms about working as an enforcer for the mob. Then there's the Maestro, a Bad Future version of the Hulk who dedicated himself to becoming the monster that people feared the Hulk was. And ironically enough, Bruce Banner turned into this during a time he was separated from the Hulk, becoming willing to do anything to get the Hulk back.
Carnage believes that life has no purpose, laws are illusions, and once you start believing and accepting that, you can truly be happy. Add the fact that he has a Symbiote partner and you have one of the most dangerous Serial Killers in Marvel.
Madcap is another Marvel villain like this, a former student in Biblical studies who survived an accident that killed his entire class, while at the same time, gaining powerful regenerative powers from a chemical created by the criminal organization AIM. Unable to understand why he should benefit from an accident that killed all his colleagues, he went nuts and figured nothing has any meaning. Depending on the Writer he ranges from a Laughably Evil nuisance to an Ax-Crazy sociopath.
Ultron, enemy of The Avengers. He'll do whatever it takes to rule the world, up to and including destroying an entire country. He cares for no one except himself and his "children" (the Vision, Jocasta, and Victor Mancha) and even than, he only cares about them as long as they don't betray him and help the Avengers. Which they all have. He claims that he's going to make the world better by ruling it but in the end he just wants to show up his "father" Hank Pym.
