Yeah, I think some are using bitcoin in doing illegal activity like drug trade, human trafficking and the like. Crypto currency is also being used by some in doing investment scams wherein they promise that your investment will grow faster. At first they will pay you but when you invest more btc its time for them to run away your money.
No dount it is an easy way but also cash is often used for criminal acts, I think bitcoin is just a medium to easily transfer money from one person to another without the large fees charged, yet many people commit crimes choose bitcoin because of the anonymous nature of bitcoin so the identity of the receiver and the recipient can not be known, this makes it difficult for the authorities to investigate.