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.
The only reason why I consider your post sensible is that Bitcoin is a new kind of currency that is not yet very familiar to a lot of people. At this particular stage, it is bad if it is going to be significantly associated with the illegal world of drugs, arms, and other illegal stuff and services. But aside from that, I do not think it is a very vital point that Bitcoin is used in illegal transactions. Our old fiat money is way dirtier and corrupted as compared to Bitcoin.