It definitely didn’t come across like that’s your point, but I see it now. But no, shooting someone in the head or killing any human being in any way will ever be humane (caveat for assisted suicide in euthanasia, but that’s a different can of worms than the death penalty). How humane a given procedure to kill a person is can only be assessed in relative terms with one another. And in that comparison, nitrogen chambers are definitely way less painful and suffering inducing for the victim, as well as, arguably, ethically more acceptable for the people who technically have to set up the process and push the button. But I doubt this would sway legislation one way or the other, as the political discussions rarely touch on how humane the method is.