Warren Okuma said:
See, no, right there is the whole basis of what the problem is here.
Good and Evil are not just opposing teams. I hate it when people assume that's all the alignments are, and I see it all too often.
Good people have to actually be Good and do Good works and think Good thoughts. Just going out and killing Evil creatures is not Good. It can possibly accomplish Good by preventing those Evil creatures from going out and committing Evil, but in and of itself, killing is a Neutral act in D&D because it does not end a creatures existence, it merely changes it's state of being (As the Aiel say, killing is the same as dieing, any fool can do either.)
But killing without justification or remorse, or the intentional killing of innocents is an Evil act.
Even if there is a strong possibility that an infant creature will one day grow up to be Evil, it is still an infant that may be raised to be Good, and preventative execution is anathema to the concept of Good.
Evil beings kill other Evil beings all the time, that does not cause them to gradually shift into a Good alignment.
Orcs kill elven babies because orcs are Evil. However, it does not follow that elves kill orc babies because elves are Good, because that is a travesty of what Good means.
I'll use this example from an episode of The Real Ghostbusters cartoon (one of the best cartoons ever made, and one of the best episodes of the show) to illustrate. Good and Evil are battling over the fate of a human soul. (They're playing baseball in proxy of an actual battle, but that's beside the point.) The Evil side is cheating left and right to win. The Ghostbusters complain to the Umpire, and he says, (I'm paraphrasing here), "Of course Evil is cheating. It is the nature of Evil to cheat. But if Good cheats, Evil automatically wins because Good has then
become Evil by using Evil methods, and there is no longer a Good side, merely Evil battling Evil."
Goddang, that show was good.