Actually, quite a few have throughout history.
Nope. Societies attempt to remove, one way or another, those incapable of living peacefully with their neighbors.
Many ignore it as long as they can benefit from it. Slavery is only one such example.
It's all part of rational self-interest.
Example, if two persons are castaways on an island and one decides to kill the other in the rational expectation that since no one can know that another person was with him and he can easily hide the remains, that rationally he will exist longer with the remaining food and thus have better odds of living long enough to be rescued, would it be moral or immoral for him to do so?
You see, the problem is philosophical. Sometimes immoral things are rational and in a person's best interest. And sometimes, even if they are irrational, they are right or wrong.
I'm not calling you a liberal, (i'm truly not) but your philosophy is kindred with theirs.
The Bible specifically addresses slavery as a good thing. If God says it's okay, then it must be moral, huh?
Your example is, of course, an extreme one and not likely to affect society as a whole. A similar situation to what you describe actually happened to the crew of a whaling ship in the mid 19th century. The upshot was that, before they were rescued, a couple of them had been killed and eaten by the rest. No one was prosecuted for the killings (the victim's were chosen by lot) or the cannibalism, as the pragmatic and very Christian folk from these whalers' homeport felt the men did what they had to do under the circumstances.
Do those circumstances hold true for society as a whole? Nope.