Stewie. Several facts of his life have been severe excuse of why he's a psycho. He claims to have lost his sanity while he was aware in Lois' womb.
Death. The poor guy is despised and treated like shit by everyone for doing his job and can't form a good human relationship. Even Peter takes pity on him.
Roger Smith from American Dad!. He is so evil because his species releases a bile that kills them if they don't "let their evilness out". Made worse when it is revealed the reason he is trapped on Earth is that the others of his species wanted to get rid of him. In addition, there are moments where he really seems to care about his adoptive family. It is implied that Roger only acts that way because he was made to be evil, and not by choice, and if you stop to think about it, it's terrible being him.
Carl the Evil Cockroach Wizard from Yin Yang Yo. It is implied on numerous occasions that his failed attempts at being a villain efficient and physical abuse of his older brother, is resentment that motivates him to try to defeat Yin and Yang to impress his mother.
Satan from South Park, especially in The Movie. He often comes off as just a nice, friendly guy who tries to act tough, and all he really wants is to move up to live on Earth (even if that will doom humanity). Just listen to his so-calledVillain Song, "Up There."
Also Butters in season 6. After the main boys snub him, he adopts the alter-ego "Professor Chaos", and plots to drown the world... with a garden hose.
Scott Tenorman, the Big Bad of the 200/201 episodes. He went insane after Cartman made him eat his own parents, which led to his plan to steal Mohammed's power to not be made fun of.
Trent Boyett. The reason he wants revenge on the main characters is that he was put in juvenile hall due to doing something they asked him to do, and when it had terrible consequences he got all the blame. They convinced him to light a fire in the classroom (so they could play "fireman" and put the fire out by urinating on it), but it quickly burned out of control and their teacher was horrifically burned; Trent was sent to juvy and the main four weren't punished at all. Butters also saw what actually happened but he wouldn't speak in Trent's defense, so Trent wants revenge on him as well.
Ezekiel Rage in Jonny Quest: The Real Adventures, particularly in his first episode, where he is a delusionally tortured man bent on revenge for the loss of his family.
An incomplete Invader Zim episode involves a giant creature called Squishy, Hugger of Worlds, a childish titan whose planet hugs eventually destroy them. According to plot ideas, Squishy is constantly tormented by other races (including Zim's species) and hugs planets like a sad child clings to a blanket or teddy bear.
Blackarachnia from Transformers Animated. She really didn't want to be turned into a half-organic mutant, and only wants to be restored to her original form and re-accepted into Transformer society (namely, with the Decepticons; she no longer trusts the Autobots). However, the methods she resorts to are unethical and often dangerous, and her old friend Optimus knows that he has to stop her hurting anyone.
Wasp is another example. He was falsely accused of being a Decepticon spy, sent to Autobot prison, driven insane by his time there, and forcibly transformed into a Techno-organic by Blackarachnia. However, he is a homicidal maniac, a Decepticon, and wants revenge on Bumblebee, who didn't know that Wasp was framed. When he found out, he eventually apologized. The newly-dubbed Waspinator's response?
Waspinator: Wasp...forgive...Bumblebot...but Waspinator NEVER FORGIVE!
Flame Princess is also one. She had her heart broken by Finn (Jake in disguise) after he changed his mind about Finn dating her since she's evil. He did this just after he sang a song telling how much Finn likes her. After this, she goes Yandere and goes on a rampage. She probably doesn't get many boyfriends due to her unpredictable personality and the way her dad seems to shelter her because of said personality.
The Earls of Lemongrab nearly became this in "All Your Fault".
Lemongrab himself. Just look at several tragedies in his life: Parental Abandonment, gets insulted, assaulted, and bullied by his own mother, called by his mom a "butt", and most recently witnessed the death of his son Lemonjon, which made him the tyrannical dictator in "Too Old". He probably sees this as Unstoppable Rage in his pain. This is more evident for people so emotionally unstable/sensitive as Lemongrab, seeing your little boy die right in front of you is not an easy thing to deal with. Fridge Horror: When describing their profound joy about their newfound fatherhood, Lemongrab describes this feeling as "so pretty okay." He has been chronically depressed for so long that he's unable to adequately articulate what it feels like to experience true happiness. It has also proven to be a very depressed and suicidal person.
Gargoyles has Demona. To be fair, a lot of it is her own doing, but that poor girl has suffered. Losing almost her entire family, being abandoned by everyone she loved and trusted, and being hunted for a thousand years by a family who is extremely fond of Disproportionate Retribution? That'll mess anyone up. The only thing that keeps her going is her insistence that it's Never My Fault; if she ever stopped denying the truth, she'd implode. The episode 'City of Stone' (from 4:10 onward) gives us a very good example of how badly Demona is broken.
The Sisters: You must give them the code. Demona: (In a trance) I will have vengeance for the betrayal of my Clan... Vengeance for my pain. The Sisters: But who betrayed your Clan? And who caused this pain? Demona: (Getting agitated) The Vikings destroyed my Clan! The Sisters: Who betrayed the castle to the Vikings? (Note: It was Demona.) Demona: The Hunter hunted us down. The Sisters: Who created the Hunter? (Note: Demona did) Demona: Canmore destroyed the last of us... The Sisters: Who betrayed Macbeth to Canmore? (Note: Demona) (Pan to wide-eyed 'What have I done' look on Demona's face) Goliath: Your thirst for vengeance has only created more sorrow. End the cycle, Demona... give us the code... Demona: (Tears forming while saying slowly) The access code is...alone.
The DCAU's rendition of Ace from the Royal Flush Gang. She's a scared pre-teen girl whose massive Reality Warper and Master of Illusion powers have wrecked her life. She's first trained by the government, then joined The Joker and the Gang, and then not only do we learn that Joker wants to use her to kill/mind rape everyone, but she ends up snapping and mind-raping him. It ultimately turns out that Ace's powers are killing her and she could potentially kill everyone in the whole city she's in as she passes away; and this happens in days when she's so desperately lonely that she has warped a part of a local park into a creepy house and kidnapped people to try making friends. Amanda Waller then tells Batman to give her a Mercy Kill before she has her fatal meltdown, but when confronted by him Ace asks him to "Stay with Me Until I Die", he complies and she dies quietly by his side, which keeps her from destroying the city. Batman then carries the kid's lifeless body in his arms, visibly and genuinely saddened by this.
Spider-Carnage, of Spider-Man: The Animated Series. An alternate version of Spider-Man, he watched first Uncle Ben, and then Aunt May die in rapid succession. The creation of his clone, Ben Reilly, only worsened his mental stability, and the revelation that he, and not Ben, might be the clone drove him almost completely over the edge, leading to an attempt on the latter's life. What firmly pushes him into this trope though, is what happens following his possession by the Carnagesymbiote. Driven out of his mind, Spider-Carnage combines the Spot's portal technology with a disintegration bomb that will destroy not only his own world, and not only the Universe, but thetotality of the multiverse. As he puts it: "I'm gonna destroy all reality". Clearly insane, and running on pain and angst, he's eventually snapped out of it by a talk with another universe's Uncle Ben, leading to a last minute Heel-Face Turn, followed by suicide.
Mr. Freeze from Batman: The Animated Series was splashed with cryogenic chemicals while fighting with his boss, who he'd been stealing resources from to find a way to cure his terminally ill wife, Nora. He was even willing to kill innocent people to get revenge. This interpretation of Freeze proved so popular that it spread into most other adaptations of Batman.
In The New Batman Adventures, Freeze's condition had worsened, reducing him to a living head without a body. Nora was revived but left him for one of her doctors. A comic tie-in expands on this. Nora's new husband, jealous over her love for Victor, hid the letters that Victor wrote to her. When Nora found out, she tried to return to Victor but could not accept his criminal past.
Terry: Freeze, you've got to get out of here. The building's about to collapse. Freeze: Believe me, you're the only one who cares.
There are other villains besides Mr. Freeze who qualify for this trope. For instance, there was Calendar Girl, a former model who got too old for the business and now seeks revenge on those who forced her out or failed to restore her youthful beauty. She is later revealed to actually be gorgeous under her White Mask of Doom, but:
Aang isn't evil or villainous in any way, but it is scary when he gets upset and goes into the Avatar State.
Hama. Kidnapped and imprisoned by the Fire Nation army as a teenager, prevented from bending her native element (water), it's no wonder she took her only chance of escape, by turning the guards into People Puppets by bending their body fluids. But then she snuck into the Fire Nation and started kidnapping innocent civilians with the same technique she used on the prison guards...
In the Sequel SeriesThe Legend of KorraTarrlok and his brother Noatak, better known as Amon turn out to be examples of this flavor of Anti-Villain. Their father was Yakone, the infamous crime lord that Aang had defeated and Depowered decades before the start of the series. Yakone proved to be a Horrible Father, perhaps almost as bad as Ozai. He forced Tarrlok and Noatak to learn bloodbending so that he could use them to exact his promised vengeance against Republic City. Ultimately, Noatak turned on him and ran away from home, with a festering hatred of bending that would one day lead to him starting the Equalist revolution under the guise of Amon. Ultimately, Tarrlok kills himself and Amon by igniting the fuel tank of Amon's escape boat using an Equalist shock gauntlet.
Yivo from Futurama, always faced system incompatibilities when he tried to court the Futurama Universe. Philip J. Fry got him interested.
Scooby-Doo on Zombie Island gives us Simone Lenoir and Lena Dupree, settlers who came to what would become the area around New Orleans two hundred years ago, presumably to escape religious persecution. Their commune, by sheer bad luck, fell under attack by merciless pirates, who fed everyone except them to alligators For the Evulz. Making a Deal With the Pagan Cat God for enough power for a Roaring Rampage of Revenge, they found that they had been cursed with immortality and the need to suck out souls to maintain it. It's quite jarring that, at the end of the film, they get the same punishment as their accomplice Jacques.
Frank Welker personally thinks that his character Megatron from Transformers might be one of these, mentioning several times in his interviews that he asks himself why Megatron is so angry all the time, and it might be because there's "a lot of hurt" inside him. It's somewhat played with in the IDW comic adaptation of Transformers Generation 1 and in Transformers Prime; there Megatron was a miner turned gladiator who suffered the harsh life of the Cybertronian working class, living under the corruption of the Autobot/Cybertronian high society, until he decided to change the system to a more equal variation in peaceful, non-violent ways, only to meet bloody supression. This forced the idealist to change his tactics to more violent ones, but the war slowly turned the idealistic rebel into a bitter and angry warmongering tyrant.
The Real Ghostbusters - One of the more character-driven episodes "Ragnarok and Roll" deals with Jeremy, a man angry at the world over breaking up with his girlfriend, Cindy, and vowing to bring about the end of the world. He nearly succeeded as he brought Ragnarok into full swing and the Ghostbusters were unable to stop him. Had Cindy and Jeremy's companion, DyTillio, not talked sense into him, the Ghostbusters were ready to detonate their proton packs in an attempt to stop Jeremy and the end of the world.
Teen Titans features Terra, a former friend of the Titans who was turned to the dark side by none other than Slade.