Ilivais X's protagonist, Iriana Estchell, is this all the way. Her life essentially boiled down to being converted into a Wetware CPU by an abusive instructor who loved taking advantage of how pleasure is felt as pain for her, interspersed with more flat-out rape which was meant to promote her love-based Drive Core to consume her. She finally escapes with Seyne's help and has in her possession an advanced Super PrototypeHumongous Mecha powered by a Perpetual Motion Machine. "Destroy absolutely everything in the Aztec Empire" is now #1 on her list of priorities. There's only the slight problem of how she's not really THAT good at it, and said Aztec Empire isn't inherently evil at all.
"All I ever wanted was to someone laugh on my jokes, just once, but no one ever did. So I decided if I couldn't live my dreams, than I take everyone else dream by ridding the world of bending."
Worm: Several villains who have done disgusting and horrible things have some truly shitty backstories such as how Riley became Bonesaw, Bitch's entire life till the point she met Taylor, and the things Heartbreaker forced Regent to do as a kid.
Poor Scion. And what Taylor does to herself and everyone else to stop him.
How can anyone not want to give Dr Insano a hug after his plans go wrong?
The Nostalgia Critic seems to have a Split Personality problem, so you honestly do feel bad for him when he looks like he might cry after realizing an explosive tantrum of his blew up a city.
Ma-ti, a semi-regular character of TGWTG until the third anniversary, has become this in the year between the third and fourth anniversaries. He literally became a cosmic entity hell-bent on wiping out the whole TGWTG universe through a decision he forces upon the Nostalgia Critic.
There's also Margaret, the girl in Linkara's Magic Gun. The gun was created by Margaret's parents torturing and killing her as a sacrifice to their God, and in exchange she drove everyone who took up the weapon who was responsible for her death to madness and death.
In Alfred's Playhouse, Dictator Alfred is the Ax-Crazy megalomaniacal evil half of Alfred's Split Personality. Alfred was sexually abused as a child, and imagined himself in a happy fantasy world to escape what was happening to him... But it turns out that Dictator Alfred ended up being concious for him and had to take the abuse that Alfred couldn't. He ended up being very cold and vengeful because of this, and now wants to Take Over the World and force everyone to obey and love him. Yeah, it's that kind of series.
In The Blackrock Chronicles, Rythian runs the risk of becoming this, after Zoey Proasheck leaves to go elsewhere and Duncan Jones and Sjin harass him. While dangerously destructive and nearly as bad as Duncan and Sjin were (for context, their mini-war pretty much destroyed the first Yogscast Tekkit server), there's no doubt that he's gone through a lot of hardship.
Red vs. Blue has one in the form of their Arc Villain for Season 15, Mark Temple. Originally part of the oldest groups of Sim Troopers to exist - and in fact, the template that was used for the Blood Gulch Crew, Temple was forced onto the blue side while his childhood friend Biff was placed on the red side. They hatched a plan where Temple would shoot off Biff's finger to earn him a medical discharge and a way to return to his pregnant lover, only for Carolina and Tex to show up and play capture the flag with their teams, with the pair of troopers following them to get witnesses for the finger plan. Suddenly, Carolina grabs Biff and uses him as a human shield. Tex then throws the flag pole like a javelin at Carolina, who deflects it ... causing Biff to get impaled through the stomach and die. The revelation that the Sim Troopers were essentially slaves to Project Freelancer caused Temple to lose his sanity, recruit the surviving Sim Troopers and go around killing everyone involved with Freelancer.