Some of you might think I'm a jerk, but these threads are always entertaining to me. The lengths some will go to, in order to convince themselves they are not taking a life! Oh my!
Don't fool yourself. You are killing something. You are taking them from Chicken to Former Chicken status. How complicated can that be? As complicated as you make it. Of course I wouldn't make the process drawn out and painful, but you're making it more painful on yourselves and probably more so on the chicken as well by over thinking and over doing it. Dry ice? Shooting it full of starter fluid? Are you serious? Having witnessed human deaths, I can tell you, there is no easy, painfree, oblivious way for it to happen. The organism knows it's dying no matter how it happens. Nor is there true "dignity." Unless you want to take it to a vet or otherwise shoot it with just enough pain medicine for death to somehow happen. Even then, it's very debatable whether there is panic or not once the animal or person realizes they are about to die, since drugs that keep the brain from being able to make the body move will change the outward appearance of the event.
Just do it. Quick. Quit trying to make it so that you don't have to think you're taking a life. You are. Do it. Me, I do it via cutting the jugular and letting them bleed to death. It's really likely that simply cutting the whole head off is just as "peaceful." Oh yes, you see the body go into convulsions after, but that's nerves firing after death. Turn around and don't look if it will bother you.
No, I'm not an unfeeling monster. I cry often when one perishes if it wasn't meant for meat in the first place, or if it didn't live long enough to fulfill that destiny. But you have to take responsibility somehow.