Native Americans were warring and killing each other. They don't get a pass.
As far as Islam, they, ISIS, are trying to restore their caliphate. This is who they are. Read the koran. It states that they must kill the infidel or other non converts. There is nothing difficult to understand of their intentions. Mohammed preached the sword while Jesus taught peace. Again, this isn't difficult to understand.
Seems that if people fight among themselves then it's okay to commit genocide against them, huh? I guess it makes it easier for people to sleep at night then. I wonder how the Nazis slept at night, oh, well the Jews sometimes kill each other, so it's okay then, zzzzz.
Also, what is preached and what is written are not the same thing.
http://www.mechon-mamre.org/p/pt/pt0520.htm#16
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13 And when the LORD thy God delivereth it into thy hand, thou shalt smite every male thereof with the edge of the sword;
14 but the women, and the little ones, and the cattle, and all that is in the city, even all the spoil thereof, shalt thou take for a prey unto thyself; and thou shalt eat the spoil of thine enemies, which the LORD thy God hath given thee."
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16 Howbeit of the cities of these peoples, that the LORD thy God giveth thee for an inheritance, thou shalt save alive nothing that breatheth,
17 but thou shalt utterly destroy them: the Hittite, and the Amorite, the Canaanite, and the Perizzite, the Hivite, and the Jebusite; as the LORD thy God hath commanded thee;
18 that they teach you not to do after all their abominations, which they have done unto their gods, and so ye sin against the LORD your God."
Oh, well, let's teach this, that you should kill all the male enemies, take the women for your own because, hey God told you to. This is an example, but it's still besides the point.
The point is, if Jesus preached peace, as you say, then how is it possible that Christianity has been one of the most brutal religions in the history of this world?
Take the Conquistadores of the Spanish in the Americas, or the British in the Americas, Africa, Asia and Australasia. Deeply religious people going around the world killing. Look at saints, many of them were killers.
You might have a religion which has a religious book that says stuff, it doesn't matter, it's how people interpret this. Now, in times when they feel under threat, they're going to be far more aggressive than when they're not. And they must feel under threat because, hell, they're a lot to be worried about, usually the USA invading. Why do you think Iran wants nukes? To stop the US thinking it can invade. Why does the US want to stop Iran getting nukes? So it can invade if it feels like it.