Capital punishment is a thorny issue, but your arguments are loaded with misunderstandings and fallacies.
OP wasn't calling for harsher methods; they're concerned about making a bad act seem "better."
Yes, abolishing the death penalty is relevant. It doesn't directly answer your question, but it pushes the conversation toward action, not just analysis.
I think you twisted Chetzemoka's words. Whether "extreme pain" or "peacefully," we're still talking about killing someone, not just "allowing" them to die, so let's not phrase it that way.
Let's focus on constructive debate, not misinterpretations and logical dead-ends.
To directly answer your question, nitrogen shouldn't be worse, and as a matter of fact should be loads better. But we don't know with certainty, so the argument is that makes it automatically inhumane.