Simply because an orc is an orc, does that make him a bad person? Or is it the orc that raises that orc to be that way. Suppose there is a 3rd option, to not kill nor simply spare, but to raise it up and teach it good from evil. Some people act in evil simply because what they are doing they view as the "right thing". Like the Nazis thought they were performing good. But most of us and the rest of the world saw was an act of evil. Its all truly opinion when you get down to the dirt of it. Not saying the other side is right of course, because I see that as evil and what we here do in the name of chivalry is good. Do you get what I am saying?
I get that you might be looking for a more solid answer- but for me it is simple. Orcs should be put to death the moment they draw breath.
All kidding aside, you make a valid point- what's evil (or not) is up to the observer to decide for themselves; that Hitler thought he was in the right did not make him so. He preached and practiced racial genocide on an obscenely large scale- granted, he's not the only one to have done so, he's the example you chose so I'm using it. But you may be right; our societal mores are drawn from how we're raised and what we're taught- but that does not absolve us from culpability; just because you're wrong and don't know it, doesn't mean you're off the hook. Ignorance is not an adequate defense.
I'll leave you with a quote from Kingdom of Heaven that sums up my thought perfectly:
"None of us know our end, really, or what hand will guide us there. A king may move a man, a father may claim a son, but that man can also move himself, and only then does that man truly begin his own game. Remember that howsoever you are played or by whom, your soul is in your keeping alone, even though those who presume to play you be kings or men of power.
When you stand before God, you cannot say, "But I was told by others to do thus," or that virtue was not convenient at the time. This will not suffice. Remember that."
