Determine Your Goals
You may be looking for a humane way to pre-kill mice for pet snakes---mice can't be poisoned if they are to be eaten. Or you may be looking for a natural way to rid your home of pesky mice. The method you choose may differ, depending on whether you need to kill a captive mouse or two at a time or an entire population.
Start With a Cat
If a cat or two suits your lifestyle, there is no more natural way to kill mice inside your home. The cat does not necessarily have to be an enthusiastic hunter. Quite often, the mere presence of a cat will send mice in search of a safer environment. Cats aren't necessarily humane when killing a mouse, and they don't always eat what they kill, so you may need to clean up the occasional rodent carcass.
Live Trapping and Quick Killing
Live traps allow you to catch mice alive and either relocate them or kill them quickly. Live traps are available in most hardware and home stores or you can make your own at home.
Natural Killing Methods
There are several ways to kill mice without using cruel traps or poison, some more humane than others. Dropping a mouse into a bucket of water to drown could be considered natural, but it is not a fast death, and neither is placing it in a freezer.
Severing the spinal cord causes instantaneous death. This can be done by placing the rim of an empty metal can over the mouse's neck and giving the can a firm smack, or grasping the mouse's head with one hand, the torso with the other, and quickly pulling apart. Cleanly decapitating the mouse with a sharp knife also results in an instant death. Putting the mouse in a pillow case and swinging it hard against a wall usually causes quick death by inflicting massive trauma.
Hands-on methods are the fastest, but may be the most difficult to get used to. Mice can also be killed quickly by placing them in a bucket with about an inch of water and between one half and one pound of dry ice (available at party supply stores). Put a lid on the bucket and let the dry ice melt for about twenty minutes. Place the mouse in the bucket, now filled with carbon dioxide gas. This renders them almost instantly unconsious and they die painlessly within minutes.