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.