"You can't be nice to 'them'."
"You can't expect 'them' to act like you and me."
"'They' will take your kindness as an opportunity to kill you."
"Life isn't worth as much to 'those people'."
"Fight them over there or you'll have to fight them over here."
How many times have you heard justifications like this. It's why it is OK to invade their homes and kill them as "insurgents" or "terrorists" when they resist your "help".
To some people, anyway.
Well, people often treat you as you are treating them. Even if all the above excuses are correct, it doesn't prevent you from being civilized, yet prepared to defend yourself without notice. And it doesn't justify you behaving as you claim 'they' behave.