Yes , bitcoin is being used by many criminals. Bitcoin is a very good platform to perform some criminal activities by opportunists. The identification of bitcoin user is hidden so many potential crimilnals use the bitcoins in order to get ransoms after hacking or doing some other criminal activity.
Criminal activities associated with bitcoins has led to the negative publicity to a great extent i.e one of the main reason why governments are not ready to accept bitcoins as a mode of currency.
Till the time bitcoins will stay criminal activity is going to stay with it as the anonymous feature of bitcoin is inveitable to all this.