This can't be true,there are other things that make people violent, you could attribute a percentage to rap but not just that.
Well I'm not sure you can say that all rap songs are violent, maybe you could say that gangster rap is a little violent, but I don't think it's fair to say that rap as a whole is filled with violent lyrics.
Let's put this into perspective, Some Hard rock, Punk rock and Metallica songs have violent lyrics in them but they regarded as violence inducing songs. Rap music isn't always violent, if anything, these days it's more along the lines of bragging and talking about women so no, rap music as a whole cannot be the cause of violent behaviour.
If anything I think it's the other way around, gangster rap can be attributed to violent behaviour. The hip hop greats Tupac, Biggie, Dr Drey, Ice cube and Easy E were all youths who grew up in violent neighborhoods and communities and as such their music was greatly influenced by this.
Neither rap nor gangster rap causes violent behaviour, if anything, gangster rap is simply a product of it.
I hope this helps.