Board Thread:Warhammer 40k General Discussion/@comment-25618639-20150807010020/@comment-5832352-20160715040149

Could just be disinformation or propaganda. Thousands of years, civil war, lots of nut jobs out there. Possible the truth of it was distorted.

Also possible that the emperor was arrogant enough to think even he could stave off any sort of corruption or issues from making a deal with the chaos gods. Who knows, maybe the gods of chaos blindsided him. They knew they couldn't affect him directly. So they allowed him to make a group of super children that they could take advantage of.