Nitrogen Asphyxiation - 99% Nitrogen gas flow at 1.1 atmospheres through a breathing mask and wait till the brain waves go flat.
All natural, no blood splatter or gore, no drugs, inexpensive, can be done anywhere, by anyone, and painless. The condemned just goes to sleep.
Or just fill up his chamber with carbon monoxide and he just goes to sleep forever. Quick, cheap and painless.
People have been offing themselves by this method for a long, long time now.
Yup. Or any other inert gas, FTM.