Yeah, it’s common to use ‘psychopath’ or ‘sociopath’ as a generic term for a broad range of ‘crazy’ types, but there’s actually a lot of difference in the various disorders and how they manifest. I dated someone who I’m pretty sure had severe, unchecked Borderline Personality Disorder, and it was a wild ride. BPD basically makes it so that a person doesn’t really develop their own distinct personality, a lot of their emotional understanding gets stuck at a childlike level, and their emotions are extremely intense (like 3rd degree burns), and they develop some really extreme and strange coping mechanisms for it.
In practice, it meant that anything could set off a flaming-hot, days-long shouting argument. She got furious at me one time for telling her that she should do whatever works for her and not worry about me (she decided I was being controlling by implying that without my approval she wouldn’t be able to make the decision). She blew up over things like me making dinner but not the side she wanted, what an item on a menu meant, me asking her what she didn’t like about a plan she wanted to cancel, how to fold laundry, turning off some unused speakers, and me leaving her alone when she asked me to go away (I was supposed to know she didn’t really mean it). And it wasn’t just blowing up over small things briefly and then forgetting it, every single dispute was a Relationship Threatening Issue and she would use extremely aggressive argument tactics (interrupting, demanding answers, nitpicking word choice, and of course shouting) because she felt that if she didn’t win then it would All Be Over.
Projection is a (attributing what you want/do to the other person) really common defense mechanism and meant that I’d often come to her to talk about a problem and she’d preempt me by accusing me doing the thing she did. It also meant that she would decide that the problems were never the result of anything she did, it was all something I was doing wrong. Similar to that, there were a number of times where she would be in a bad mood, and then would keep asking me why I was in a bad mood until the constant questioning got me into a bad mood, then use my now-ruined mood as proof that I was lying when I said I was fine. Push-pull behavior like “I hate you I’m breaking up with you, oh my God I love you how can you walk away from me” is routine for people with BPD, and she did it a lot (I think we had a dozen and a half ‘break up for day or three then get back together’ incidents before the last one). Her mood would also flip at a moments notice, you never knew if you were going to get the fun, silly girl who you loved, or the raging harridan who said you were terrible and shouted at you.
Until I had the experience, I didn’t realize mental illness could be contagious, but the combined effect of lack of sleep (she loved to argue late at night when I had work), frequent argument (and constant preparation for it), gaslighting and projection, push/pull games, and everything else really messed with my head. I started doubting my memory and wondering if I was doing stuff and not remembering it, I would be unpleasant around other people because I was always braced for an argument, I would get mad at little things because all of my reserves were gone, and in a lot of ways I lost sight of exactly where the boundary between us was.