I've never heard of this "Bully" game so I can't give an opinion on it, but it sounds like it's appalling.
However, with the GTA series I'm much more familiar. My 13-year-old little brother likes the series a lot. I haven't played the games myself and I don't think killing a few fictional prostitutes has made him any more violent than he was before.
As Thrik pointed out, the series is rated for adults only. It's not the developers' fault if parents let their kids play games not meant for them.