The Chaos Primarch's loyalty
I am mostly looking at the Big 4 who fell to individual Chaos Gods. Before the HH Fulgrim and Magnus were extremely loyal to the Emperor, but they got 'tricked' into Chaos. Angron and Mort pretty much hated the guy from their first meeting. After "Betrayer" where it shows Angron being bat shit crazy well before the HH, even threatening to take the Emp's head when confronted by Leman Russ got me thinking.
I am betting Mort has a legit reason for hating the Emp., and part of it will have to do with the Emp. saving Mort's life. (Only Vulkan had something like that happen, and he stayed loyal).
It makes me wonder. 2 brothers hated him from the start, the other 2 were as loyal of sons that could be found. What makes it more interesting is that the Angron/Fulgrim and Mort/Magnus hatred reflects the Chaos gods that they represent.
On that note, Horus had his hands full keeping all of these factions fighting for him. I think a good book could be of some loyalist Alpha Legion donning the armor of the Traitor Legions and attacking Horus's allies. How do you think Angron would respond if he got attacked by a few hundred Emp's Children? I am sure becoming a Daemon Prince has not helped his temper.