I'm not a blank slater; I don't believe that we're born innocent, and only develop a dark side after bad tendencies are engrained by evil capitalists, or evil patriarchs, or evil warmongers, or evil whatevers. I think that, though we're not naturally racist, we're naturally "groupist." Evolution seems to have inclined us to readily define whole groups of people as the enemy, after which we can find their suffering, even death, very easy to countenance and even facilitate.
But when it comes to defining this enemy–defining the "out group"–people are very flexible. The out group can be defined by its language, its religion, its skin color, its jersey color. (And jersey color can trump skin color–just watch a brawl between one racially integrated sports team and another.) It all depends on which group we consider (rightly or wrongly) in some sense threatening to our interests.