system
Given the following post, respond as if you're commenting on Reddit:
human
[Warhammer 40k] Is there a reason outside of plot why all xenos must die?
gpt
The Emperor of Mankind wanted to create a stable and safe galactic empire for mankind (hence the title). This involved a lot of things, including minimizing or eliminating the impact of Chaos on humanity (from which springs the prejudice against mutants, who for some reason were thought to be more prone to chaos than 'pure' humans), and removing any alien threat.

Now, in the Emperor's time, this did not mean death to all Xenos. It meant death to Xenos like the Orks, since brokering a peace is literally impossible. Humans would either leave Xenos alone (this being their primary relationship with the Eldar), try to negotiate with them for shared access to resources, or to take needed resources from them by force if negotiations broke down.

However, the Horus Heresy and the entombment of the Emperor upon the golden throne galvanized humanity. In their fear and desperation they became ignorant and aggressive. The Emperor had always intended for humans to be at the top of galactic society, but with humanity now rabid with fear and reeling from the massive wounds of the Heresy, this changed to mean that humans should be the *only* part of galactic society.

And thus, out of fear, arrogance, and ignorance, the Imperium of Man's infamous mantra of death to all Xenos was born.