What is the Alien? It is merely a creature of non-terran origin. Such a thing would hold no holy disgust for us. The Xeno is an idea, a concept made deadly by virtue of its mere existence. The very possibility of the Xeno becoming the dominant force among the stars is corrossive to the Human soul.—Daenyathos - "A Thousand Foes"
No human could understand the thoughts that went through the mind of the alien.
On one level, they were driven by the same desires that defined the actions of all living things. To survive. To achieve, and attain superiority. To stave off suffering. But below that layer of instinct was something else entirely, something that was made up of both pride and desperation, an obsessively free will and a grim pre-ordained, almost biological imperative to prey on lesser species. The alien was monstrously proud, and yet he acted out of fear. He was cruel to the point of abstraction, as if the infliction of suffering fulfilled an aesthetic ideal - and yet every cut and kill was in deference to forces it could not control and that it had to bow before, no matter how it refused to accept the domination of another. A human could not possibly understand all the contradictory layers that made up its consciousness. All they ever saw were the results: the dead, the enslaved, the broken.
The more I learn about these aliens, the more I come to understand what drives them, the more I hate them. I hate them for what they are and for what they may one day become. I hate them not because they hate us but because they are incapable of good, honest, human hatred.—Inquisitor Agmar on the Tyranid hive
He who allows the alien to live shares in its crime of existence.—Inquisitor Apollyon - "The Unceasing Struggle"