This does not sound far fetched to me at all.
I find it very credible that people who regularly experience hatred (for whatever reason) or excessive anger and rage are dysfunctional and that their brain activity would function differently from a happier person's.
I know when I have experienced rage I usually experience extreme stress and discomfort just after (and probably during) the incident. I mean the incident will literally make me feel physically ill and exhausted and I have to recover from it. I cannot imagine feeling that way all the time.