Board Thread:General Discussion/@comment-27130793-20160212011843/@comment-11139677-20160212012536

I believe Chara implies in their speech in the genocide route that the player guided them into omnicidal maniac territory. If anything, I think the the main difference was that before their death they showed little malevolence towards monsters, and primarily directed their efforts towards punishing the human race-rather than killing '''absolutely everyone. '''

So yeah, they were always evil, they just get progressively worse.