The problem is one of progressive’s own creation; bleeding heart liberals who are desperate to save criminal scum make mountains out of every molehill in the execution process. The reason the government can’t use the safe and effective drugs veterinarians use to put down animals is because public pressure campaigns instituted against manufacturers of those drugs (of course, by death penalty opponents) have prevented those drugs from being sold to governments that use the death penalty. This then forces the government to use marginally less effective drugs, and of course progressives are happy to complain about a problem that they themselves created.
The real issue is that the debate is on false pretenses. Criminals didn’t give their victims painless deaths, and so the state has no obligation to do so either. But the debate has been co-opted by one side who argue about whether the drugs are perfectly painless, when we really should be discussing whether murderers and traitors deserve comfortable deaths at all.