Maiming any animal in that way is pretty horrific. I'm not sure how you justified that considering your hang-ups on feeding f/t are (have to go with Bruce on this one) personal and selfish.
Your reasoning doesn't add up, and if it were me, I would refuse to maim a creature like that, feeder or not. When I have had to feed live, I have treated my feeders with the respect they deserve as another living creature, no matter their place on the food chain. Sure they're dead either way. But I will not allow them to suffer that way by MY hand. awful.