I think it's basically why were such a successful species, our innate hatred of everything thats not like us (including others of the same species). It's no coincidence that where ever we travelled once leaving Africa our arrival coincided with mass extinctions of mega fauna and other types of humans (Neanderthals in Europe and Homo Erectus everywhere else). I was watching a program on the box over the weekend that mentioned we are even born naturally aggressive and it's just through parental conditioning (and learning) that some of us don't remain that way.
Personally, I feel sorry for all the aliens out there and not just the ones on our planet
! Apart from the Martians, that is.....I hate their big green bobble heads!!