Yes we have to face this bitter truth that bitcoin is being used by criminals for buying illegal things like guns and weapons or to pay someone for doing something.Criminals love using bitcoins because of its anonymousity.It keeps their identity and location hidden,so they have no fear of being caught by someone.But yeah as the OP says there too many good things about bitcoins too.So one can't just oppose bitcoins because it is being used by criminals.We should just accept the fact and move on that everything in this world has a good as well as a bad side.