Bitcoins are used for every kind of thing and yes off course there are also people who are not doing good things with the bitcoin so they use them for illegal shit. But i think that most of the bitcoin users are legal.
We can't identify if the person we are talking here are the ones who are using bitcoin for good or illegal activities. And there is no doubt that bitcoin was being used in illegal activities, because many people in the dark web are using bitcoin for their illegal transactions. So that's for real that bitcoin was being used for illegal activities whether it is in dark marketplaces or not.