Oh yeah I agree, but I don't think of psychopathy as the nature incapacity to empathise, but as a learned method of turning off empathy and remaining in that state so long it becomes a habit.
You are of course right that it doesn't necessarily mean they stay that way or that it isn't just a factor of the situation. I think just how difficult it is for me to read it and connect to that kind of violence also makes me want to imagine it's atypical and not just a behaviour elicited in certain circumstances.
I'm glad you recovered from that experience though, it sounds quite horrid having to live that way for any amount of time.