1. not all physcopaths harm aniamls, a great many love and respect them and wouldn't hurt one ever even if they would hurt a person.
2.He could be anti-social rather than a physcopath.
3. He might not have any disorder- does he use drugs?
4. I forget if you said his age- maybe he is immature/aggressive/impulsive as a result of his age, through our teen years we grow some thingy over our brains they call this milineation and it has to do with our maturity.If you want proper spelling and proper/full info on that google it.
5. Is his behaviour always in line with times he has high stress?
6. He could have some other disorder but only a trained professional can actually diagnose anything not just some girl who wants to say things about him.