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-37536803