I know what you mean... I stopped listening to Tool for the same reason, but I think I've learned something since then as well. Perhaps a question will help - what happens to you when you listen to them? What impact does it have on you? What words are you saying when you rap along to them? Not just talking about Bone Thugs, but any music.
As we pursue God one of our greatest struggles is "being transformed by the renewing of our mind". We grow up thinking certain things and acting certain ways because of the culture, people, influences, etc. around us. It is not easy to stop thinking the way you have always thought - you have to realize what Truth is and align yourself with that.
So, if you find that listening to certain artists pulls you into thinking the way everyone else around you thinks, then perhaps that could be your litmus test.
If you struggle with violence and don't see why its wrong, then I would say you shouldn't be listening to things which promote violence. Replace "violence" with your personal struggles and then read the sentence again to help you know if you should be listening to them or not. This is not a perfect test, but may be helpful.