Spoony calls this the “Prisoner Dilemma,” the idea that the adventurers encounter an orc village, kill the warriors, and then are left with the problem of whether or not to kill orc women, children, and babies. The orcs fail their morale check and surrender. What should the players do? [By the way, there is a standard philosophical dilemma called The Prisoner’s Dilemma — it doesn’t apply here]
This happens a lot in my game. I don’t have alignments in the game, so it is up to the players what to do, but typically they allow the women and children to flee if they face no current threat. If they are vampire babies, they will typically be killed.
In a game with alignments, it also comes to the relative threat or evil of the monster in question. As I’ve said before, good is not stupid. But good players are often merciful, especially if they are in a campaign where they are powerful and good usually wins. It is like Maslowe’s hierarchy of needs. If we don’t need to kill them, and we can easily defeat them if they try it again, then we don’t have to kill them. If they are dangerous monsters that will kill others if we don’t stop them now, then they should die. And it is perfectly ok to do so in that question, and even Lawful Goods can do that.
Unless, of course, one believes that Lawful Good means Lawful Stupid. There’s 32 pages of people arguing about this at http://www.enworld.org/forum/showthread.php?187573-The-quot-orc-baby-quot-paladin-problem.