Board Thread:Warhammer 40k General Discussion/@comment-96.237.238.119-20131218020230/@comment-24383134-20140110031250

Well Horus lost to a corrupted/posessed Planetary Governor, and Horus was supposed to be the best of all the Primarchs. Theres also the fact that some of the Primarchs have been defeated by mortals, or at the very least wounded gravely. Magnus the Red was severely wounded by Ragnar Blackmane when he threw the Spear of Russ into the former's eye. It's entirely possible for the Primarchs to lose to anything, apply enough force and anything will give. This isn't to say that the Primarchs arent awesome, even the least physically powerful of them was a BAMF, but they could still be killed.