He still knows what right and wrong is but still chooses to do bad stuff. You can make him like let’s say hurt himself or bash his head against the wall because to him it gives him pleasure. Make him hurting himself his main motivation to hurt other people. Show him being given the opportunity of doing good things but they choose not too because they love doing there psychotic stuff. That’s how I will do it

@DoctorEvil45 I think the way I would do it is that I they are aware that they're indeed a maniac, but just doesn't care and is okay/finds pleasure living with their breakdowns. Or they can have the ability of calming themselves down, which can also find them having the ability of becoming a good person (which they won't, obviously).

@Baklasagne12 there is also a another way show them that they know what the difference is between good and evil like they view good as boring and evil as fun or what @DoctorEvil45 said