I can tell you why, but only because I have an older brother who I'm sure, if he wasn't overweight and very religious, would have been a serial killer.
The brother I live with said once, when he and Andy were kids, a nest of baby birds fell from the tree. They weren't sure what to do with the babies, and Andy went into the barn and slit their throats one by one. My live-in brother had to hear them scream in pain an horror.
Andy also used to like collecting snails, carefully peeling off their shells, sticking them in a jar and when they were aaaaaalmost dead, pouring salt onto them.
Slitting the throats of mice sounds like similar behavior. Torturing small animals is psychopathic behavior. Why is he doing it? He's doing it because he likes doing it. He gets enjoyment out of torturing the mice.
Avoid the crap out of that guy.