Sociopath, possible but unlikely, he sounds more to me like a sadist, especially because he uses pain in others to relieve stress. It depends if it turns him on or not. Serial killer in the making?...

Very good and fun, especially with the adrenaline that's involved

I'm not sure how sadism works but as a masochist being hurt in a sexual way relieves stress. Maybe it's the same for sadism except hurting others. She might not think of him as a stress ball but ...

This is literally my fantasy as a sub guy, is that normal?