From recent news in India it is heard that bitcoins are used to sell drugs..I know its a currency and it is also used as a normal money... but these can create a bad space in people's mind that bitcoins are used to sell drugs..so they will not use bitcoins... Conclusion is that BTC is also a currency which is having a high value in the market..and bitcoins must not be used in these illegal activity which can reduce its usage and value.
Bitcoin cannot use for illegal because in bitcoin you can earm money so, they don't need to do illegal crimes, using bitcoin can be a payment system so maybe they can use it in illegal crimes but I don't believe that they will use it because bitcoin has a secure and safe system.