Obviously nobody is saying rap CAUSED this
but if you don't understand that different types of music has different types of energy and attracts different types of people, I don't know what to tell ya.
NOBODY should be surprised that violence occurred at a concert where the artists routinely rap about killing black people.
If this was a Foreign Exchange, J.Cole, Talib Kweli,Common,etc concert then yes I would be surprised. Their music has different energy, hence bringing out people with different energy/mindstate.