They had come from their hiveworld to attack us, not for our resources or our territory, but because of a deep-seated hostility caused by the fact that, in their species, males outnumbered females fifty to one. As a result, all their males grew up doing little else but killing other males-- of all species, not just their own. After annhilating all other life on their own world and several nearby systems, they had come to Earth and began killing us all off-- everyone, I think, because they maybe hadn't figured out the difference between boy and girl humans yet.
They were a little bit robotic and a little bit insectoid, and a whole lot of destructive. Humanity was quickly reduced to living in sewer culverts and subway tunnels, London of the Blitz for everyone everywhere. C and I were hunkered down in some portion of some dark, drippy tunnel system packed with other people but not far away from us, in a side room that I think used to hold power-generation equipment, lived a pack of goth chicks, feral and pouty ennui-creatures in fantastic clothes.
One day, when there was a break in the perpetual boom-thoom of the above-ground shelling, they announced that they were going to try to bring about peace by teaching the invaders the difference between boys and girls, and trying to reduce that competitive killing instinct by giving them some females-- that is, themselves. So somehow they went out and captured a bunch of the aliens and dragged them back into their side room and shut the door and proceeded to have themselves a sort of xenorgy, and to be perfectly honest I don't think they were actually doing it to bring about peace, I think they were just bored with refugee life and looking for something new to do with their genitalia. And judging from the horrific caterwauling that echoed on and on through the ventilation system, I'd say they'd found it. Seriously, warring forever seemed preferable to having to listen to it another minute.