Warhammer 40k

I’ve been working my way through The Horus Heresy, The Horus Heresy: Primarchs, and a few different modern series like Cain and Vaults of Terra. I just started A Thousand Sons in HH but have read up to Jagatai Khan in HH:P. I’ve also got a loose sense of modern Inquisition, psykers, and chaos (albeit tainted by Cain’s descriptions in many cases). I read Magnus the Red probably before finishing the opening HH trilogy (mostly because I’ve tried to start HH a few times and have read up to book five before).

I don’t understand what I missed in Magnus the Red and Battle for the Abyss that should have made me realize Magnus would go hard for chaos. Almost immediately when we meet Magnus in Thousand Sons it’s clear he’s going to go too far, he’s too cocky, and he doesn’t respect the warp. Before that, he really seemed like a sympathetic character, misunderstood by fellow Astartes, but in line with the level of warp you see the Imperium use in 40k. I’ve got a few questions.

1. Did I misread Magnus the Red? Are there chaos signs there?
2. Should I have not been sympathetic to all the jabs about the Council of Nikea?
3. Was the hope that Horus placed in Magnus, maybe even Magnus’s visit during his Nurgle thing, a clue I should have recognized?
4. Am I supposed to like Leman Russ more than Magnus and side with Russ in his distaste? (The whole Canis Helix thing in Leman Russ which I read first confused the fuck out of me when I read the flesh taint scene with Russ in Thousand Sons).