Not only bitcoin but all the valuable things are always used in crimes as a weapon. Earlier days people used to get robbed for gold, silver valuable articles etc. Now they got cheated by fake currencies, cheques or get robbed while doing online trading. Current rate of 1 bitcoin is Rs.3,12,788/-  no cuerrency have this much value so its obvious that all illegal activists get attracted. They can sell or buy things online & pay by Bitcoins because they are not traceable. They can ask money of ransom in the form of bitcoins. We cannot stop them.