Fentanyl has become a relatively cheap way of upping the strength of whatever opiate you're selling. It used to be that no one knew how to illegally make fentanyl. Those dogs are long gone.
But there's another aspect to this: if someone overdoses (even dies) from something you as a drug dealer have sold then (as dark as it sounds) that's actually good marketing. That means you have the good stuff. It's shown to increase demand.
The moral of the story is don't get addicted to opiates.