Board Thread:Warhammer 40k General Discussion/@comment-3416287-20150401101244/@comment-2253300-20150422235314

Actually the IMperium has a spiritual impetus behind their rejection of mutants. There is a belief within the Imperial Creed that states that the state of ones soul is tied to the state of one's flesh. According to the Ecclesiarchy the human form is sacred, and any deviance from the baseline human form is seen as a sign that the soul is corrupted or deviant somehow. This belief is not surprising given that worshipping Chaosd tends to cause physical mutation.

Anyway the degree to which one deviates from baseline human form is taken as a sign of how divrced a mutant of Abhuman has become from humanity. Not all Abhumans are accepted by the Imperium. Ogryns and Ratlings are not too far off from baseline humans, just a general difference in size and intelligence for the most part. However Abhumans that diverge more wildly from the norm are exterminated with extreme prejudice. Regardless no mutant or Abhuman could ever hope to be more than a second class citezen in the Imperium, as the Imperium's religion takes their physical abnormality as proof of some kind of spiritual taint or deficiency.

Anyway since mutation is seen as a sign of spiritual taint among the Imperium it's likely that they would view Xenos that possess a number of mutation to be particularly loathsome as their mutations are proof that they have degenrated even further from other Xenos. Just a theory mind.