Personally I think the only true way to kill any animal and be human is to cause instantaneous brain death. Suffocation is a terrible way to die for any animal and the only reason I think anyone thinks its humane is because you dont have to deal with watching them die yourself and there is no blood or gore.
Mind that I dont care how they died, I'm not them, I don't have to experience it. As long as my snakes have a cheap, ready source of food that can be bought and stored in bulk and will cause no problems for my snakes, I'm happy.
Death, in and of itself, is a terrible thing, no matter how you slice it. But at least suffocation leaves you with intact bodies and no means of the rodent hurting your snake.