I feel like a murderer :(

Like the title says, I feel like a murderer. I have mice that I raise that are food for my snakes. My snakes are not full grown so I have to kill the mice before they reach adult size. The first time I did it I had 12 to kill, lucky Chango called me and I was able to do it as I talked to him. The second time I wasn't so lucky to have someone to talk to as I did the deed. This time I have almost 30 to kill. I have been feeding them to the snakes two at a time, but still there is a bunch to kill. Its not easy killing the mice, they are a living thing and to hear them squill and gasp for air. And their eyes, you can see the life drained. They go from this bright red to a dull grayish red. The worst is when I have to do it. What I do (yes this is graphic) is I take them with my tongs by the base of the tail, put a flat head screw driver against their neck, pull on the tail so the body is tight and quickly yank up towards the ceiling, folding their neck in half. If done right this kills them right away. But sometimes I do it wrong and there they go gasping for air. No matter how many times I do it I feel so bad. Now here's the funny part, I can take the live mouse and put it with the snake and feel nothing, no remorse. I watch them kill it then swallow it whole. But when it comes to me doing it it almost tears me apart. I think the only way to fix my solution, and when we get financially settled, I am going to buy a co2 canister and make a kill chamber so I can gas them. I think then I will not feel like a murderer. (hopefully)