That's because in real life we don't have any other sentient beings to genocide. There's animals we can extinct of course but it's just not the same :(
Not so sure. When it comes to human hatred, big differences, petty differences and made-up differences work just as well as one another to motivate atrocities. Elves and aliens would end up on the list, but I don't think they'd hold us back from being awful to ourselves. Ultimately there's no real logic, so in what order that would happen is anyone's guess.
True story. When the colonists showed up in the New World, you think the natives would've all banded together against the "alien" threat, but instead the invaders were often able to leverage historic fueds and tribal animosities to get the different groups to help fight each other. I at least know this happened in some instances in South America and Africa.