So I guess that means you wouldn't have any problems killing members of another tribe, like say Iraq?

That depends entirely on what your definition of "another tribe" is. It was (and is) common practice to demonise those groups against whom you wanted to whip up popular fervour.

It's easier to kill someone who's different.

A book I've just finished reading: "Berlin - The Downfall 1945" describes the level of demonisation happening on both sides of the German/Russian front. One comment from the Soviet soldiers upon entering Berlin is that the Germans "were just like them".

Groups are defined by their differences. If you can emphasise those differences, then the other group becomes less like you, and easier to kill.