Why not use a (wait for it...) gas chamber?
Too complex. I realize that there has to be a certain amount of ritual involved, to sooth the delicate feelings of your average Liberal. However, with nitrogen, building a gas tight can big enough to keep witnesses safe is expensive and not required. They were using hydrogen cyanide with gas chambers in Cali, and a leak there would wipe out any number of "witnesses" getting off on watching someone die. A little nitrogen leaking around the edges of a mask won't be even noticed.
I'm biased. I think the death penalty should be reserved for politicians. We would have cleaner government or smarter crooks.