Yes, bitoin is used on many illegal activities like buying on the blackmarket sites... Use to buy killer etc..it is widely use in the underground transaction in china.
Bitcoin isn't any different from how fiat is being used. It's money, so it does get used for all kinds of things. And yes, also things that are not right. But we know that, and once again, just like fiat is being for illegal things, we can't do anything about it.