Personally....I feel paying for any illegal activity with BTC is a very bad idea. It is way too easy to track.
There are exceptions...for example buying BTC w/ cash from a btc ATM. But in this day and age, I am not sure why either side would want to use it. (for illegal activities).
Since it is going mainstream, the AML / KYC is lifting the veil off crypto. (which is a good thing since I believe in crypto long term). I just think it is a terrible way to do illegal things.
They will be making more and more examples out of crypto used in illegal activities. Already happening. The block chain if explorable to anyone. So much easier to track vs conventional money. (again, which is why it is good) Just not good for illegal purchases in my opinion.