This discussion always drives me nuts but I am going to take the bait then step away. First, anything that is killed should be killed in the most expedient way possible preferably without the victim knowing it is going to die. Nothing should be tortured before death.
That said, things die so other things can live. Immediate total destruction of the brain is going to be the least painful next to being unknowingly bled out. To those pushing gas, I am not by any means saying it is the wrong way, but in my head at the least we are talking suffocation. That hamburger or chicken that you just ate was not gassed.
Of all the methods mentioned the snake's own natural method is by far the most violent and painful. Constriction is going to hurt and so is envenomation when it is delivered by an animal that is appropriately equipped. If I (I have never done this) squeezed a rat to death it would be considered animal cruelty. If the snake does it, is not? If I injected a rat with nerve toxin it would be cruelty, but if the snake does it, it is not?
The arguments around this subject often defy logic and very often reflect people's own personal definitions of right and wrong rather than reality.