Board Thread:Warhammer 40k Novels/@comment-8881468-20161212151338/@comment-94.2.225.225-20161212171052

As to Horus initially believing he could take over from the lying, bloodthirsty, power-hungry Emperor, it's how Chaos works. After Davin, Horus thinks he's acting for himself -- his genocidal attacks on worlds are actually not much different from those he carried out during the Great Crusade: those who disagree with the new order, cease to exist. (As a Primarch, he's engineered to do whatever needs to be done, regardless of how horrible. Thank Big E for that one.) He genuinely believes that the Emperor is setting himself up to be a new god in the Warp (and might actually be right, in a roundabout way). Horus isn't a madman like Kurz, Angron or Fulgrim, and he fully intends to become Emperor and tear down his father's edifice of lies.

But everyone who Chaos gives the apparent power to act alone to sooner or later discovers that they're a slave to the whims of the Warp. While he might nuke a world, then smug-faced Erebus comes along and tells him it's much better to impale them all on stakes, better they can summon warp storms and daemons. Out of strategic necessity, Horus begins to comply -- and eventually finds himself doing such things as if he believed that all along. He is slowly but surely manipulated into murder for its own sake, to spread fear and suffering throughout the galaxy, which empowers the Ruinous Powers. By the time he gets to Terra, he's long since lost the notion he's doing this for the good of Mankind and just wants to rule the galaxy for his warp-bound masters.

The newer canon makes clear that Horus went to Molech and did exactly what Emps had done in the Age of Strife and used its gateway to the Warp, to get the same power. In that moment he became not only the champion of Chaos, but the Chaos version of the Emperor himself. Molech is very close to Terra, so chances are it was the last warp jump before the Battle of Terra. I would consider that the moment Horus throws off the last pretence of being a rebellious warmaster trying to through off an unjust ruler.