I used to live in an apartment building that had mice, and my housemate and I would catch them in live traps and release them way out in the woods. We always worried this sort of thing would happen, so this cuts deep. I mean, not enough that I didn't laugh, but still.
It wasn't meant to be. If he had kept the mouse, it would have died by electrocution, overeating his pellets, or possibly having its tail chopped off by a midwife.