That audiodrama kinda confirmed my thoughts for the longest time - the whole "It was planned" thing.

It just seemed strange, and so odd, that The Emperor would treat certain sons this way, and other ones so coldly and callously.

His treatment of Angron was the most glaring. It was almost as if he judged him right from that moment on that this son will rebel, and he'll just nudge him on that path.

Conversely, the way he treated Magnus, Sanguinius, Vulkan, Horus, Fulgrim, Gulliman, Dorn, Russ - was like a proud dad who pins a medal on his kid, or plays catch with them in the afternoon.

It was almost as if He already knew, and had plans outlined for them.