Warhammer 40k Fluff Discussion XVII: Call Necrosius, The Old Thread Is Dead!

[Wraith]

It's one of those plots that is a mix of out-of-universe story telling, and in-universe mythology.

We won't find out for sure until the duel is finally written into a novel, but there are reasons as to why the Emperor would hold back and not kill Horus, that later got romanticised as being 'love for his son'.

Maybe it takes a few minutes to fully and properly unhook himself, psychically, from the Golden Throne - he can't just switch it off, and Loken(?)'s sacrifice literally buys him the 10 seconds he needs to gather the full limit of His power for the killing blow?

Maybe the Emperor still thinks he can rehabilitate Horus, but in a more neutral way - Horus is still the most powerful of the Primarchs and he now knows things about Chaos that no-one else does, so perhaps the Emperor offers him one last chance to surrender and use his knowledge to restart the Great Crusade? They chat for a while, Horus eventually refuses, they fight while the Emperor continues to insist that they can put the Heresy behind them if only Horus surrenders?

Maybe the person who stood between the Emperor and Horus was Sanguinius? Sanguinius is holding Horus down and the only way for the Emperor to kill Horus was to run his sword through the Angel too? Hell, maybe Sanguinius talked it over with Horus and decided that actually, letting Chaos win would be for the best - Horus and he were close confidants before, if anyone could talk Sanguinius around then it might be Horus.
Either way, nobody needs to know that the Emperor was willing to kill Loyalist Primarchs - or that Sanguinius had Fallen - so they instead make the story about how Sanguinius was already dead when they got there and it was compassion for some rando Guardsman that let the Emperor overcome the enemy instead?

Maybe the entire duel is a lie. Maybe Dorn or Valdor walks in on the Emperor and Horus both badly wounded and he can't tell them apart, so he kills them both and concocts a story about the Emperor's great and noble sacrifice rather than let the Imperium know just how similar the Arch-Heretic and the Master of Mankind really were at the end?