I would be interested to know more about this, because I've seen people say (on forums, I've not been involved directly with heroin use or users), that fentanyl is pretty evil stuff, and it's commonly reported by them as something that's sold as heroin to the unsuspecting addict.

Dealers pass it off because it's easier and cheaper to get, because suppliers find it easier to move. I've never heard of users deliberately going after it.