Just a few thoughts to stir the pot...
I like to explain orcs by socio-cultural reasons, mainly because I loathe the idea of a race being ultimately evil (manly because I hate the idea of good and evil
). The moral values and upbringing methods in an orc society differ much from what we call acceptable or normal or good. In the same manner a human child (hopefully) learns from his surroundings that killing and hurting others is wrong, an orc child learns that it's right. The surroundings from which an orc child learns are originally created by Sauron or Morgoth or whoever, and repeated by other orcs around the child because they have been brought up the same way. Similarly the children of the free peoples are, I would think, taught to hate orcs.
So what is the difference? Why are the heroes never bothered by the amount of slaughter
they do, or rather, why are we never bothered by it? Why are
they still not evil in any way? Okay, they don't take joy in violence and they are on the defending side...
But somehow that isn't sufficient for me. True, we could justify the heroes' hatred for orcs by what orcs have done to their families or homes or freedom or whatever, but aren't we then doing the precisely same thing that those people who eg. hate all supporters of a certain religion because some 0.00000001% of them are terrorists who happened to kill someone, or those who hate all inhabitants of a country because their leader is a brute?
Of course LotR is an epic fantasy novel, almost like a fairytale, and I'm probably making a mistake in comparing it to things happening in this world. After all, Middle Earth is a world of its own, a fantasy world, and in such places I suppose good and evil can exist even if they didn't in this one. (Especially if it is a symbolic story or especially if the author needs a way for describing war in his books without making his heroes seem cruel because they kill others...
)