Lethal injection and C0 poisioning are both very VERY painfull ways of going, if science is to be trusted,
You sufficate to death with the lethal injection, C0 will make you sick as hell if you get the mix of air and c0 wrong,
The best way to go would be to go in one of those chambers they use for divers, where they suck out the air,
You would feel drunk and silly, then you just die. it would be amazing,
All you have to do is go up to 50000 feet and try to die by asphixiation before you freeze to death. or get one of those chambers.