There was no way to perform the execution correctly. N2 asphyxiation makes no sense as an execution method -- it will only be painless if the person didn't realize what was about to happen or if they were cooperative. If not, it'll mean the person will fight until death. They will be put in a situation of endlessly expanding pain until they can bear it no more.
The idea that an execution method is humane if it would be painless for a cooperative victim, ignoring the experience that will be had be an uncooperative victim, is just... really dumb. It's incredibly dumb.