system
Given the following post, respond as if you're commenting on Reddit:
human
[Warhammer 40K] Why are the Humans/Space Marines in Warhammer 40K so xenophobic?

I understand that races like Orks and Tyranids are constant threats to humanity, but why so hostile generally?
gpt
Before the age of the Imperium there was the golden age of technology in which humanity ruled a vast empire. 

During this time, the human leadership allied themselves with several alien species, trusting them and treating them as friends.

Yet when humanity's empire colapsed their "friends" turned on them, not hesitating a second to enslave and exterminate trillions of humans for thousands of years untill the great crusade ended them.

The Emperor did not forget and he did not forgive. The earliest and most holy laws decreed by him at the start of the great crusade teach the Imperium to kill the Xenos, purge the mutant and never to allow the creation of Abominable Intelligence. On pain of death.