It is not always irrational, if anything it is rarely irrational. It's a survival instinct that has come from thousands of years of ingrained experience. The only person who has a vested interested in your continued existence is you, the more degrees of separation from you, the lower that interest becomes.@Lament I mean, humans are a bunch of bastards that often hold irrational hatred towards anyone not in their group; we've seen it almost constantly throughout real world history, so it shouldn't be surprising that it'd get taken to an extreme with actual non-human races.