So I've seen a lot of stuff on the internet about people's opinions on how best to dispatch the rodents for either fresh or frozen pre-killed food, so I figured I'd put in my $.02 for this little mini article. To give you guys some background, I'm a grad student in biology (emphasis on environmental and reproductive toxicology) and I've probably directly or indirectly killed about 2000+ mice in my career as a grad student. Here is the way that I have found to be the most humane:
***NOT FOR THE FAINT OF HEART!***
If you have access to carbon dioxide or dry ice, you can make a "charged" chamber by filling a bucket or some other container with the gas directly or put in some chunks of dry ice. Let the dry ice sit for a few minutes to let the gas (which is heavier than air) sit at the bottom of the bucket or tank or whatever. Place the mouse or rat in and hopefully the charged container will knock the mouse or rat out in just a few seconds. They will usually pass out, release their bladder, and kick for about 10-30 seconds. When they are barely breathing or completely out, make sure to dispatch the animal using some other method like cervical dislocation (see below). I have seen mice which I thought were in mousie heaven come back after what looked like was their last gasp! Especially if you freeze your food, it is no fun to wake up freezing cold in a freezer!
If don't have access to CO2 or dry ice, cervical dislocation is probably the next best. I personally think picking the mouse or rat up by the tail and whacking it hard on a table or something is quite cruel and you might just severely injure it and not deal it a lethal blow. Instead, I prefer to get a blunt object (the dull side of a butter knife or the closed blade of scissors works fine). For juvenile animals, hold the mouse/rat gently but firmly by the tail so that it is calm and minimizes struggling (you don't want to miss because it is freaking out), put your instrument about 5-7 cm above the target, and quickly give it a blow about a centimeter right behind the ears. It should have broken its neck quickly and the animal might kick a bit with the nerve reflexes. Check to make sure that the neck is indeed broken. For adults, you can do the same thing, but pull at a 45 degree angle backward on its tail just after you hit its neck (keeping the blade of the scissors or whatever on the neck to hold the body down) and you should hear the vertebrae separate. I wouldn't try this on mice younger than 3-4 weeks since you could pull off their tail.
An alternate method I've heard is getting a bucket, putting a live mouse/rat trap in the bottom, and placing the mouse/rat inside. Listen for the snap, look to make sure you don't have to finish it off, and collect the prey item.