Well, that particular group of demons were allowing their home to break free of any bindings, and insofar as the text mentions, there is no indication it wasn't a willing contract. That sounds to me like both a good thing and not the type of demon that forces inhabitance -- hell, even when they left it wasn't them that did it, but christ that let them into the pigs. Apparently they couldn't go without permission.
As for the self-harm and screaming, well, apparently the guy was at it for a while. I'm not one to support cutting, and a noise disturbance et al, but apparently the fellow was considerate enough to move pretty far away from people before starting his racket, despite people trying to tie the guy in chains, and wanted to do so badly enough he was willing to put his body on timeshare to make it happen. Doesn't quite sound like the demons were the one that caused the mental torment -- enabled it, maybe, but not caused.
I have to say, though, I love the propaganda-like makeup this is taking. Both sides trying to paint the other in the worst possible light, and overtly put words in the quoted text's mouth to do so. Mind you, you two are reading a lot more into the nature of the demon than the anti-cat side, but well, that's the sort of mental gymnastics that occurs when you're trying to excuse the death of a couple thousand sentients.