Good to see I wasn't the only one disappointed with this book. Everything from the Blood Angels portrayal, to the Emperor/Thousand Sons conversation, to Mortarion's apparent 180 about the warp, the Slaanesh corrupted Knights etc. had me cringing throughout the novel.
Pretty much the only interesting parts for me were the bits of history about the Emperor and how he gained his power, and how Horus gained similar power to confront him. Horus was reasonably well portrayed as well, unlike a few other novels where he just seemed angry.
The human appendix. Proof of a higher power. A divine kill switch so to speak.
No one really likes a smartass, but people tend to like a dumbass even less.