Everything is used by criminals, not just Bitcoin. For sure, the great majority of the criminals are using fiat. The more dangerous ones are definitely using fiat. They are the ones who are wearing suits and are occupying high positions in the governments of different countries and in the top executive positions of various international companies. In the low level, even scissors and frying pans and pins are used by criminals as well.
Right. Not only bitcoins they are using, but if they are using almost is fiat. There are many people who are using bitcoin for illegal doings, most of them are hackers, because if you are a hacker and want to pay something, you will be needed bitcoin because in bitcoin, you can receive or send money without providing your personal information.