I'd suggest that the reason identity is so important is that people through the ages have linked the existence of a tribe (religion, ethnicity, whatever) to their sense of immortality. If my tribe goes on and survives then in a way I will live forever. When you attack the tribe, you attack continuity and foreverness (is that a word?). Unfortunately, humanity has not yet formed a single cohesive tribe. Maybe we need to be invaded by ETs or something.