It's not killing everyone - it's dominating everyone. Killing everyone would be Monoextermination (... which should totally be a Monodominance sub-faction). I feel that sometimes (and I may have to do something about this, as I really like this idea) Monodominance is very much focussed on the Xenos and the Mutant, and also on the religious aspect, but there are other, still brutal, options. Here's a good Monodominant-style quote -
“There will be no loyalty, except loyalty towards the Party. There will be no love, except the love of Big Brother. There will be no laughter, except the laugh of triumph over a defeated enemy. There will be no art, no literature, no science. When we are omnipotent we shall have no more need of science. There will be no distinction between beauty and ugliness. There will be no curiosity, no enjoyment of the process of life. All competing pleasures will be destroyed. But always—do not forget this, Winston—always there will be the intoxication of power, constantly increasing and constantly growing subtler. Always, at every moment, there will be the thrill of victory, the sensation of trampling on an enemy who is helpless. If you want a picture of the future, imagine a boot stamping on a human face—for ever.”
The Imperium does not just seek dominance over the alien, but also over the human. The Emperor rules absolutely, and His agents rule below him, but because the Emperor's rule is absolute, so, as His agents, is theirs. Dominance is everything.