I'm amazed no one has mentioned what, to me and lots of other herp keepers, is the quickest and most humane method of pre-killing-cervical dislocation. When you do it properly, death is intantaneous, by far the fastest I have seen of any method. CO2 doesn't even come close.

It's easy to accomplish. Holding the mouse gently by the tail, lower it onto a surface on which it's feet can readily gain purchase-a screen tank cover works perfectly. The rodent will naturally attempt to pull itself away from your grasp, stretching itself out slightly. At this point, gently bur firmly place a pen, sexing probe, or other long cylindrical object just behind its skull, while at the same time giving a short, sharp tug to the tail. This will easily detach the cervical vertebrae (and thus the spinal cord) from the base of the skull, causing instant death.

It's never pleasant to have to take a life, but such occasions do come up. As responsible keepers we must strive to do our utmost to prevent suffering of ALL our chargers-whether they are pets or feeders.