I can't say I'm much of a fan of rap, but it has occurred to me that it might be a spinoff from the old talking blues, which is a form that's been around for a long time.
As for the content, I agree with some of the posts above — the blues seemed to have an element of hope, of coping with adversity, but rap seems to be more about creating adversity. Maybe the kind of lashing out it advocates is a way of coping, but it's a short-sighted way at best. If urban blacks really started to do what the rappers urge, I think they would find themselves the victims of "ethnic cleansing" that would make Kosovo and Somalia seem like a stroll in the park. Racism against blacks is bad enough, but open black racism toward whites just compounds the situation.
As for the music that accompanies rap lyrics, I find it repetative, mechanical and annoying in the extreme. It seems almost devoid of any human content — unless you can find the human content in monotone disco with overlays of the above-mentioned trash compactor.
Still, if people want to listen to the stuff, that's their right. But their rights end where my ears begin, so they can darned well TURN IT DOWN.