As a veterinarian, I've euthanized hundreds, if not thousands of animals (last 30 years of my career spent as a diagnostic pathologist... all my 'patients' arrived DOA, or I had to put them down to work on them.
I've never understood the difficulty these states have in terminating death row inmates by 'lethal injection'. Put in an IV catheter... they do it millions of times a day in hospitals around the world... bolus in a dose of barbiturate-based 'euthanasia solution' (you're essentially giving an overdose of an time-tested injectible anaesthetic)... they should just 'slump' almost immediately into unconsciousness -> lifelessness, and at most give a deep terminal breath or two. Finis.
None of this messing around with all the various 'cocktails' of paralytics and potassium chloride, etc., some of these states have used.
Better yet, a well-placed .22 round would be effective. And cheaper than anything else I'm aware of - though the cost of a quick high-voltage/high amperage jolt may be cheaper... depending upon local electric utility rates.