I think it's a bit simplistic to blame rap music, but that is one facet of an ugly, thuggish culture. It is possibly as much symptom as cause.

In my opinion much of it stems directly from deep personal inadequacy and a distorted sense of self-esteem (or just self), where the only recognition or gratification they receive is from the fear of others.

For the underlying causes of this, you would have to look deeper and I'm sure there is any number of papers or theses on the matter.