There are harmful, fringe, uses of those items but the vast majority of the use-cases fit a slow cooker aren’t harmful. The same cannot be said of bitcoin, where the primary use-case is facilitating nefarious business.
In Bitcoin that use case is wealth transfer that is uninhibited by law.
That essentially makes it kind of like an index tracker for the underground economy.
I don't disagree that it's an underground economy, but that's only because it hasn't hit the mainstream. But being underground doesn't necessarily mean nefarious or illegal. Those bad actors definitely exist in the Bitcoin economy and all other economies. But without some data I'm having trouble coming around to the idea that the majority of Bitcoin is used for illegal purposes.
I find it hard to believe that anything beyond a small minority of transactions are the "good kind of underground transaction".