Normally, I wouldn't (And don't) criticize any genre of music seriously. There's a lot of music I don't like, but there's very few genres that I can say I hate entirely. I make fun of country the most, but it's generally just in good fun. And there is a rare good country song here and there, and an occasional Rap artist I like: Mix A Lot is too funny, Eminem was okay until I started to find him insufferable, and RZA actually have a lot of talent. It would take somthing special for me to take real exception to a genre at large.
However, the problem with rap music, and I don't say this lightly, is racism. The KKK wish they could be as toxic to black culture as Rap music, or rather more accurately, Gangsta rap. First of, Gangsta Rap is carefully presented as being a piece of black culture, and when you associate things with black people (At least in America), Rap is always of the list. Then, of course, is the content: Shooting people is fine if it gets you money, hit your girlfriend, steal stuff if you want it to get ostentatious trinkets, and do whatever immoral activity you feel like doing if it benefits you, regardless of the price those around you pay for it. It's a terrible message, but it's not the problem. We watch violent movies and play violent video games and read violent books all the time, no negative effects. But that is because those things acknowledge that there in this entertainment only space. It's done for fun, letting off steam. Gangsta Rap, however, does everything it can to make the fantasy real. And when you look at the rappers themselves, well a lot of times they practice what they preach. That might be why I enjoyed Eminem, because you can see the tongue placed firmly in cheek, while most other stuff is less entertainment, and more disturbingly twisted Moral Objectivism (Or no more twisted then it already is, but that's another conversation entirely). And a lot of money is being made by selling the image of the violent, selfish black man, and I have a serious problem with that.
Rap music is like if they made a GTA game where people acted realistically when they were shot or run over, and bragged about how they used real people getting killed for there 3D models.