This was our family rule. Since we were never faced with coyotes "eating our lambs", then I guess that was never expresses as part of the rule. But you are right.... self-defense is a good enough reason as well. I would not hesitate to kill a rhino charging one of my kids, nor a coyote killing my herd.
The Rino issue is easy - stay away from them unless you a serious big-game hunter. Now the coyote thingy gets tougher, as there are many "varmits" I hunt and have no intention of eating or skinning.
Keep in mind there are various scavingers out there that will take care of any downed animal. Those coyotes, ground-hogs, feral dogs and (yes) deer will be consumed by critters who take care of any dead animal (and they all do die.)
Some deer are ill or weak from lack of food and best dispatched to end their suffering. No reason to haul them out of the woods. Just an example of why sweeping statements like "eat what you shoot" don't hold up in many situations.