One common sentiment in this Chapter I think is that you guys tend to generalize. Alright, just imagine yourself being a Space Marine in 005.M31. For your entire life as a Space Marine, you are fed myths like the "The Emperor has abandoned us" like millions of other Astartes. Also, you had to put up with annoying Remembrancers and being put under civilian oversight, which you, like millions of Astartes, blamed on the Emperor. Overall, your life sucked because you went from an age of warriors into an age of civilian annoyances. Then, suddenly a Primarch comes up and promises to change everything. He promises to restore Primarch Rule. Of course in doing so, he also feeds you plenty of lies (The Emperor lied e.g). Remember, the Inquisition at the time didn't exist. You basically think what your Primarch tells you. Would you have joined him? Like millions did at the time in great numbers? Remember, the Horus Hersey had over half of the human people serving in all terms. You cannot call half of the population evil. That is pure generalization. My point is that one cannot generalize the Horus Heresy as inherently "evil" because it was joined by trillions of people. We know from human nature that people tend to join organizations that to them seem "right" and possess the moral high ground. By calling the Horus evil, you are calling humanity evil.