I have found, through trial and error, that blunt force is the best way to kill mice fast, unless you want to build a CO2 chamber. Here is a way that works well, and gets them dead fast.
1. get a plastic back like the ones that groceries come in, and target gives you, and the like.
2. drop in 1 to as many mice as you want to kill, numbers dont really have an effect on this method.
3. Hold bag by the handles securely. Spin bag around to build up inertia and get centripital force to pull the mouse to the farthest point from your hand.
4. Follow the spin and as the bag comes down in the rotation, swing your arm down, so that you slam the bag off of the ground. The inertia produced from spinning allows there to be enough force to kill a mouse. 1 hit, mouse doesnt know what happens, just knows hes dead. works wonderfully.
The bag keeps in all(if any) blood splatter that may occur. You might want to tie the bag before spinning. i did a 5 mouse bag one time and when i hit the bag off of the ground, the force made one mouse fly out of the top of the bag, and down the hall. Dead, but wasn't what I had in mind for him.
Try it, it works very well.
Truth