In the case of the first nitrogen execution, they did dick all to vent away the carbon dioxide he was exhaling, so it eventually saturated the gas he was able to breathe and his lungs wouldn't have been able to get rid of any more. When you hold your breath, the discomfort and urge to breathe again comes from the CO2 buildup rather than the lack of oxygen.

If the exhaled gas gets vented properly, then there's no discomfort. That they didn't get this part right for the execution suggests malice, or at the very least extreme negligence because it doesn't take expertise to understand this, just a little bit of depth in knowing how suffocation works. Which you'd figure people designing and carrying out an execution would seek.

Having seen firsthand what happens when someone unknowingly enters a hypoxic enclosed space, I think the difference is foreknowledge. Thrashing sounds like acidosis from holding one's breath. I was helping an acquaintance work on his old steel boat. There was a watertight compartment. The risk of steel-enclosed spaces is that rusty steel in an enclosed space can consume all of the oxygen, leaving only nitrogen rich air.

He opened the hatch and, before I could stop him, he just strode on in like it was nothing. He was unconscious before I could get to him, maybe ten seconds. Fortunately, he was near enough to the hatch that I could just reach in and grab him, rather than trying to find an air tank and regulator, and then put it on.

He recovered just fine, but had a terrible headache. He didn't remember anything about it. He didn't thrash. There was no drama. He walked in and fell unconscious. Lucky for him it was a small space, so the bulkheads kept him from doing a full header into the steel deck.

When you take a deep breath of it and die the death you’ve been wanting it’s painless. When you want to live your entire body thrashes in pain akin to drowning as you struggle to not breathe in.

This is part of the problem with “humane” execution. If you slip it into meals without prisoner knowledge death row prisoners will starve themselves to death. Start nitrogen bagging sleeping prisoners and they’ll do everything in their power to never sleep. These people want to live and have an intense instinct to survive by any means. Suicidal people can often overcome it, but everyone else will generally endure any pain for a tiny amount of time.

I think the difference is that right-to-die advocates have gone to insane lengths to ensure beyond a reasonable doubt that their setups which administer the lethal substance do so painlessly, so as to ensure that the person willingly choosing to die spends their last few moments not in pain anymore.

Prisons don't seem to have the same standard in mind with their own setups. If anything, they seem to want to maximize suffering for the sake of the spectacle they've arranged the execution around.