I thought overdosing on opiates was highly easy to achieve, basically pain free, and also relatively cheap even in the social backwaters of the world where they have death penalties?
This is how pets are euthanized, usually, and it's about as humane as it gets. That said, you'd never sell this because the portion of the population who want the death penalty either want it to be inhumane, or would balk at the possibility of making the condemned euphoric before death.
How an opiate impact a person varies from person to person. Some would fall asleep, others would violently convulse, some would get trouble breathing, and others would get irregular heart rhythms and multiple heart attacks.