It irks me that when thinking about villains in KH, you realize that characters on our side of the story have done nearly almost as much bad or were flat out worse than them, lol.
First, let's look at our main villain, Xehanort. From what we know about him, he abused his only know student, beating Ventus up and bullying him as well, and manipulated Terra throughout the events of BBS, as well as using Ventus. Then he ruined Aqua. As a student under Ansem, he did experiments on the Heart and was responsible for turning the first members of the Organization into Nobodies, as well as having a hand in sinking Radiant Garden into the Darkness (although the game says it was Maleficent who actually did the dirty done cheap here, but he's the one who opened the Keyhole of the place in the first place allowing for her to show up)
Then we have the actual asshole of Kingdom Hearts and the one know to have done the WORST out of all characters, Ansem the Wise. Let's recap what this terrible old man did:
- Kept a literal dungeon (lol) with people locked up that he experimented on, with at least one being an actual kid. While we can't be sure if the other prisioners were regular baddies of RG (which doesn't make his case better but does justify the dungeon at least), Subject X is implied to be just a regular amnestesiatic girl who did nothing wrong.
- Essentially stole the entirety of the Grid and repurposed it as his own personal computer. Tron literally says he hates him, and unlike SDG, he meant the actual Ansem, not the impostor.
- Experimented on people's hearts. He tried to stop Xehanort from going further with the research, but I seriously doubt the HUGE canyon-like laboratory filled with hearts we see in KH2 was built solely by the apprentices after they pulled a coup on him. I'm also inclined to believe the dungeon where the prisioners were kept is meant to be the hidden space that Xemnas goes throught in the extra cutscene from FM+ when visiting Aqua's Armor, not the actual dungeons we play through in KH1, and that place is pretty much a secret villain laboratory if I've ever seen one (which brings the question of how did Lea and Isa even sneak in there in the first place?)
- Treated Xion with as much disdain as the Organization, if not more. Like, Riku at least took some pity on the poor girl, but he was pretty much dead set on treating her like an "it". lol
- Everything about what he did to Roxas. He did take some pity on him and did things like showing the train that goes to Yen Sid's tower, but that doesn't excuse the way he treated him.
- He's also ok with using Riku, a 15-later-16 year old as his literal Reaper and pretty much sending the kid into a path of darkness, even if the later was ok with it for Sora. But he knew that whole ordeal was sneaky otherwise he wouldn't have had a reason to hide it from Mickey.
Sure, he regreted most of it and did some things to try to amend, but then we have KH3 where he's back to power and doing just fine in RG again when he should be in jail, honestly?
We also have the OTHER asshole, Eraqus.
- He treated Terra like garbage for something he was barely at fault for and then sent the man in a wild hunt after his former friend who he knew dealt with some sneaky things in the past, and then sends Aqua to tail him because he doesn't quite trust him. Like, why even send Terra in the first place instead of just training him further and letting Aqua go after Xehanort?
- Took Ventus in but in the first opportunity where he acts out of his jurisdiction he puts all the blame in the kid, acting suspicious of him. Again, if you are going to mistrust a literal 14~16 year old because of who he used to live with then why did you send your star pupil who was also pretty much your son on a wild hunt after him without even letting him know that hey, maybe don't trust Xehanort so much?
- Everything that happens at endgame when we return to Land of Departure. He literally goes for blood after Ven and then Terra and jesus the only other time we see someone with actual death intent like that is Xehanort killing Kairi in 3 (and the mysterious killer from UX).
- The end of KH3, where he's like "oops my bad that I almost killed both of you! but yeah my bad Terra pls take care of them. Anyways time to go on vaccay with the boyfriend, later!"
Seriously, it's like adult figures in the KH are almost always bad save for Yen Sid who is protected by Disney. No wonder Sora's mom never showed up, maybe she would be the big bad of them all.