I agree that behaviors are caused by physical states. However, stating that people who behave in a fundamentally different way aren't physically different is misguided. Every person is physically different. The difficulty is that unless there are gross brain lesions, it's very hard to see what makes people with psychopathy different. It might be a slight difference in the sensitivity of key neurons to one of twenty neurotransmitters, for example, or a slightly different pattern of connections in one region.

Psychology (like many sciences) is all about developing reliable abstractions for these complex physical states. The surprising thing is that it works: you can change the neurotransmitter balance of the brain in many cases just by sitting down and talking to people for a year.