Try thinking of it this way: any rodents raised at the various rodent supply companies have their fates sealed from the start. They are going to be food no matter what, whether it's for your snake or someone else. It's not you making the decision to kill the mouse, and you don't have to see it be killed. I would never eat a steak if I had to slaughter the cow myself, but since I can buy it in a prepared packaged form without seeing the animal die I can deal with it.