Never had fentanyl but as a recovering addict I hear it's pretty much 100% non euphoric. Give them horse instead. Fentanyl is only more deadly because it's so potent at a genuinely tiny dose but there's no reason you can't just administer, like, 50 times as much heroin. Let them go out on a high note (hurr hurr)

Much more humane. Give them a solid hit for their size/tolerance, give them five - ten minutes to come up and enjoy it, then slowly push the OD.

But they give you a smoke and it means 5 people have to do it which is good for labor.

I realise that, very few things are, but lethal injection is explicitly not even designed to kill people quickly. It appears peaceful only because the victim is injected with a paralytic. At least with a firing squad you can tell when it goes astray, and should be able to do something about it.