Do not use an airsoft gun. I've been in similar situations, and the best thing, if you have to kill it, is to do it as quickly as possible. It's never nice, but if you cannot reasonably save it, and if you have the stomach for it, a quick stomp will do the trick. When I was younger I tried killing a rat we had caught with a pellet gun, it didn't turn out well. The bugger was resilient, and took nine rounds, it was messy. Anyways, it seems cruel, but you'll feel better ending it quickly. Don't do it in front of the kids obviously.
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