Kyoka Nanjou and Yakushiji Gaito from A Drug That Makes You Dream are my go-to characters for this. I would have found them to be utterly horrible people even without my involvement with Aeka. Kyoka generally used her as a way to keep her own position in her class' faux-hierarchy, a litmus test for seeing who else would fall in line, and a scapegoat to take out her own frustrations with her boyfriend, and her willingness to do so clearly showed her twistedness. Gaito was simply a bully and a violent thug and an implied rapist, and just the first glimpse of him and his sadistic, drawling voice provoked my fight-or-flight instinct.
Because I loved Aeka almost immediately (and only loved her more as the story progressed), though, I didn't just hate those two. They terrified me, with what they could and increasingly did do to my girlfriend and me. After a certain point during Aeka's route, I was starting each in-game day feeling honestly sick with fear. That fear, that loathing, and the throat-chewing rage those eventually combined into were part of what made the game so affecting.
...along with Aeka herself, of course, but that's more for a thread like "Characters you honestly fell in love with."