As a US Soldier, you kill a suspected terrorist in a marketplace in Baghdad. Across the street a shopowner straps a bomb to his wife who blows up your fellow soldiers. Do either have a guilty conscience? Are either or both wrong?
In a terrorist's mind (and perhaps also in his society's mind), he is doing "right". A soldier that kills the terrorist is "wrong."
From the soldier's point of view, it's the other way around.
Moral relativity in action.