Due to its value and popularity, many criminals was engaged to used bitcoin as an online payment. Bitcoins may be challenging to track because of its  decentralized system and no paperworks required, so criminals can do some illegal transactions.
For these reasons, we should all be very careful in how we think about Bitcoin, because most evidence suggests Bitcoin to have the same net positive effect, if not more, than the internet. Both the internet and Bitcoin are tools that can be used by the ‘bad guys’ or the ‘good guys’, and thankfully most of the world are in the latter category.