It sounds like he is trying to introduce you to his "dark, nasty, and spiteful" side. I expect if you tolerate this, it will show itself more often. Unwanted, intrusive thoughts can be caused by past traumatic events and anxiety disorders. This becomes problematic when the person cannot distinguish pathological thoughts from reality. It also sounds like there may be an element of underlying paranoia, which might indicate a possibility of psychosis or schizophreniform disorder. You are not specifically describing hallucinations or delusions, but I would certainly investigate those possibilities. Has he ever had a mental health evaluation? Does he have a history of substance abuse? Has he been capable of sustaining an adequate school or work life? He does appear to have low self esteem and he makes poor choices regarding relationships. If this continues to be a problem in your relationship, I would suggest that he see a counselor to rule out a serious mental health disorder.