Because of the semi-anonymous nature of Bitcoin, its used in ransomware attacks. Hackers breach networks and hold them hostage until payment is made to them. Hospitals and schools have been victims of these types of attacks. However, unlike cash, which was favored by thieves in the past, Bitcoin always leaves a trail in the blockchain that investigators can follow.
But then again, the features of bitcoins are a double edged sword. You have the decentralized and uncontrolled transaction but then again, crimes can be easily committed by having people send money anonymously. But we can all see and appreciate the good things bitcoins have brought into the world.