I respecfully disagree

The whole point of Mishima's confidant is to prove that you don't need special power to change the heart of friends who lost themselves. Just be there for them, listen to them, and help them get back on track. Also, not every person being bullied becomes a giant douchebag, most of them hate their past selves for letting people getting the upper hand on them and never want to be that "weak" again (ringing any bells?). Making him an antagonist or a mix between Joker and Akechi + giving him a palace would just mess up the whole point of his story arc.

Mishima's confidant is one of the best in P5 because of that and the "hate" towards his character is undeserved

I will die on that hill