"there are millions of offenders in a few forums"

Actually, this "evil dark web" statistic is false. That is the number of accounts. It is established in news articles that people who use these sites create an account every time they access it.

One article estimated the actual number of people to be around ten thousand, although it's unclear whether that is wholly accurate.

I'd also be wary of determining the behavior of every user (i.e. to try to argue greater immorality) based on users who "speak". Remember, the 1% rule: https://en.wikipedia.org/wiki/1%25_rule

That said, it's irrelevant what some criminals do, when so many people also use these tools for legitimate purposes.