The Kingdom Hearts series has Master Xehanort. He's responsible, both indirectly and directly, for all the hardships the main characters suffer and the creation of the three main monsters that are destroying the worlds. The secret reports in Birth by Sleep paint him as a bit of a Well-Intentioned Extremist, unleashing the Heartless and the Unversed to flood the universe with darkness in order to bring about perfect balance between light and darkness. However, that's in the backstory - in Birth by Sleep, he's clearly Jumped Off The Slippery Slope and whatever good intentions he once had are nowhere to be seen.
Describing Master Xehanort in such broad terms makes him seem positively tame compared to his actual portrayal. What makes Xehanort truly awful isn't the amount of harm he causes to the world, but the depths to which he sinks in his treatment of Ven and Terra. He takes Ven, an eleven-year-old (or even younger) child, in as an apprentice with the sole intention of training him to use the darkness, then stealing his body. When Ven proves himself not to be up to the task, he forcibly extracts Ven's darkness, leaving him comatose, then drops his apprentice's failing body off in his own much-hated homeworld to die. After realizing Ven isn't going to die, he gives him to his fellow Keyblade Master Eraqus, a man whom he considers to be a brother, to train until Ven's light grows strong enough that he can be forcibly reunited with his darkness and become a weapon that will allow its wielder to reshape the entire world. While he's there, he notices Eraqus' student, Terra, and immediately decides that he wants Terra's body. When Terra's finally about to become a Master himself, Xehanort convinces Eraqus that he ought to be tested to show that he's not dark, sabotages the test, isolates Terra from his friends, and uses his position as a Master to keep Terra from learning to suppress his darkness properly - then, he uses Terra's issues with the darkness to draw Ven away from Eraqus' protection to force him to fight and gain strength, eventually letting him know what he really is and sending him off to confront Eraqus, knowing his "brother" would feel compelled to kill Ven and Terra would do whatever it took to save him. He murders Eraqus while he's still weak just to enrage Terra, repeatedly tries to kill Aqua just because he doesn't have any use for her, callously uses everyone he comes across, and is willing to risk an apocalypse out of sheer curiosity. His predatory behavior destroys the lives of everyone he comes across, and what makes him even worse is that there's nothing anyone caught in his plans can do about it - the only reason he didn't succeed completely was because he didn't expect Aqua to survive long enough to interfere, and he still ended up with Terra's body in the end, with Aqua inadvertedly helping him keep it!
That's not the end of it. He manages to convince a group of scientists into manufacturing a batchan entire army of evil beings FOR SCIENCE, while suffering from amnesia(!), throws his master into the middle of nowhere, steals his name, willingly splits himself into two(!!), his heart possessing a boy and manipulating a cartel of Disney villains from behind the scenes and his empty body manipulating said scientists (who followed his fate) into fixing something he was directly responsible for, treating them all like dirt and planning to off them all eventually anyway. The kicker? Even though Sora managed to defeat both his incarnations ten...eleven...TWELVE years later, he's now alive somewhere and whole...
Let's not forget his But for Me, It Was Tuesday reaction to the events of BBS. Ruining the lives of three aspiring Keyblade wielders (one becoming his vessel, one falling into darkness to save said friend, and the other having his heart broken) merits nothing more than a smug, matter-of-fact reaction from Xehanort.
And 3D makes it even worse. See the part above about Xehanort being responsible for his ownStart of Darkness? That wasn't Master Xehanort's doing. This guy was responsible. Lacking a body, ASOD traveled back in time and convinced a young Master Xehanort to go through the annals of time himself and recruit thirteen different incarnations of Xehanort to form a new Organization XIII. The young Xehanort then proceeds to return to his time and work his way through the events of BBSjust so he can make it back up to the point he (as an amnesiac Terra-Xehanort) loses himself to darkness (again) so that he can become a Heartless and Nobody, thus ensuring a Stable Time Loop. When Sora discovers this chain of events and inquires about it, Xehanort even admits that he doesn't fully know what's going to happen next. Xehanort is so depraved, he even used himself as a pawn in his schemes. Just when you think Xehanort can't sink any lower, he proves you wrong. And this becomes even worse when you realize that, while Xehanort, in general, is Sora's Evil Counterpart, his younger incarnation is Riku's; had none of this ever happened, he'd just be a boy living out his remaining years quietly on the Destiny Islands...
He's a murderous Psycho for Hire, but next to Master Xehanort, he's like the diet coke of evil, even considering the severity of their deeds only. The fact that he never had a choice to be anything else, while not enough to really cause pity for the guy (as, unlike say, the Nobodies, it means he was never once GOOD either, and he does have actual emotions so that he can genuinely enjoys his evildoing) does make him a lot more understandable and far less disgusting than the willfully malicious choice to be evil made by his master.
