I haven't read up on this topic for quite a while. I remember that both freezing and decapitation are considered slow, painful and cruel due to herp physiology. Organs crystallizing hurt, decapitation isn't quick because they retain awareness due to primitive nervous systems.
Better to spring for the lethal injection, any way you look at it.
I don't know how CO2 works on mammals, much less reptiles. Some sources say that the build-up of CO2 in mammalian lungs burns and causes a lot of pain.
I think I'll ask my vet about this next time I see him and get some hard answers.