While I agree that "murder" is the wrong word, there is an angle that you are missing.

Drug dealers will lace "normal" drugs with fentanyl. It gives a "different buzz". The customer knows they are buying "normal" drugs, and there is no quality control, so lacing happens. If the dealer messes up how much they lace, it is easy to add too much (because a small amount t of fentanyl goes a long way), and your customer ODs.

I’ve heard from an HSI friend that a lot more of the fentanyl deaths happen from accidental exposure in their drugs from shared surfaces. For example, somebody buying cocaine that was cut up on a table that was previously used to cut up fent and not properly sanitized.

Manslaughter is the word you are looking for, they don't intend for the person to die.

I was indeed missing that angle. Thanks for mentioning it