Heres what I do to the mice I have bred into adults.
I take them by the tail.
Place a paper towel on a hard surface, WHACK WHACK it twice.

Its basically not conscious after that even if it is still alive.. I love all animals. Your not a bad person.. Your method is a bit.. odd though.

An easy way is to put one in a paper bag and hit the bag as hard as you can on the table a few times. :]

Its quick for the mouse, quicker than what the snake does. It's also a lot safer for the snakes.