why not just flood the execution chamber with nitrogen or some other inert gas?
I think it's even easier and I'm not sure why nobody does this: Drain the blood from the person. I gave blood (once) and passed out. It was not very scary, a very short window of "oh, that's weird" and I was gone. I came-to several minutes later and was fine. No gas or dangerous environment, no pain, just drain the blood out.