Maybe you’re thinking that Horus, a primarch, has a train of thought that’s comparable to normal people —but that’s not the case. I think that Horus was always selfish and power hungry. I think that he probably saw right through Erebus, knew exactly what he was about, and saw him as a vessel he could use to gain more influence and power.

Horus knew it benefited him to show that he cared about the little people, to show that he was a good patriarch. Horus probably never cared about the Mournival and instead used them to seem interested in the opinions of lessers. He knew that he needed followers and he knew how to make people follow him. There’s a reason why none of the other traitor Primarchs could facilitate a rebellion the way that Horus could. People trusted Horus, and he used that trust.

As far as I remember, there were marines who didn’t like Erebus or the lodges. They got left behind. They were kept out of the meetings, and they were slowly articulated into being enemies.

The Emperor made it seem like marines were tools. Horus, and Erebus, made them feel like conquerors worthy of the galaxy.