In my current WIP I've tried very hard to show that the two cultures both have good and bad points and they look very odd to an outside point of view. It's just these two cultures don't blend well (they both consider the other's religion to involve 'unnatural practices')....which is a bugger when they're forced together by a vicious drought.
I love it when cultures aren't necessarily evil, but they clash anyway.
I actually don't really like it when cultures are evil, to be honest. I mean, an entire culture made of only evil people? How do they ever get anything done, what with all the backstabbing and murdering and so on that must go on all the time!
In my WiP, the magicless humans (Efira) justifiably blame the magic race (Divira) for the terrible virus that affects those with magic, but kills those who have no magic, even when it's not their fault.
Even the alien race is just looking for a new planet to rebuild its people, who are doomed to extinction otherwise. Of course, taking over the planet means devastating all previous life on it, so all three races are fighting to preserve themselves.