Some people use bitcoin to buy illegal stuff, the majority of people use bitcoin on legal things.
Some people use paper cash to buy illegal stuff, the majority people use paper cash on legal things.
Bitcoin is just a currency. How it is used is a different story.
In this piece, bitcoin being a currency and it being used to buy illegal stuff seem like the same story:
http://www.dw.com/en/bitcoin-paying-with-the-cryptocurrency/av-37536803I like the way you said it, everything can be used for good and bad, it is all depend on the user, but because there were a lot of negative publication about bitcoin, so when only a slightly illegal usage of bitcoin being exposed, the people reaction will be blaming bitcoin not the user