You can use money to buy illegal stuff, you can use gold to buy illegal stuff, so it does not mean anything if someone uses bitcoin to buy illegal stuff. Bitcoin, fiat money, gold, are just different forms of the assets.
I can't agree with you. Bitcoin is anonymous that's why it's can be easily use in illegal stuff in the dark web and other many illegal platform.
Fiat Money and Gold are very different things because these is you will never use it anonymously because those are physical asset.