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.

Fentanyl is also given to people in hospitals (in the US), getting people hooked.

A lot of times people think they're getting a different drug that happens to be laced

As Arturo Salazar said, "Addicts? They treat themselves... they overdose, and there is one less to worry about." Dark movie

So if you bought a bottle of water, and inside of that water was a poison that kills you, intentionally added by a criminal because it makes them more profit, that's not murder?

Those people were doing a line of coke, they had no idea they were about to overdose on an opioid.