Heresy from the Emprah’s point of view
Pretty much this. The Emperor was 'born' in 8,000bc, somewhere around Anatolia. Likely to be Nevali Cori and the nearby temple of Göbeklitepe. He's the living reincarnation of hundreds (or more) shaman elders, and probably has their souls shouting at him the whole time. He has no fixed appearance, and basically unlimited power (to all practical extent). He hid out, probably under the insistence of his shaman 'fathers' for thousands of years, taking a back seat and watching, learning and sometimes guiding as humanity staggers its way into the future, fuckup by fuckup. Everyone he knows is born and dies in the blink of an eye. He has no peers, no equals, nobody who can even comprehend *what* he is.
You know how when you're a kid, ten minutes seems like forever? And at thirty, two days is right on top of you? You can probably remember yesterday in full, probably most of the last week. What do you remember from when you were ten? Flashes and highlights, probably an hour's worth altogether. Now compound that out thirty eight thousand years.
So he has trouble forming relationships, because he has absolutely nothing in common with anyone, and they all just keep dying all the time. That's what comes of being the New Man.
But he's got all those dead Shamans living inside him, and they're steering him towards the good. He's passive, most of the time, because he's seen enough and knows enough that a giant golden immortal leader stepping forwards, at most points in history, would damage humanity's development more than help it. The human race needs to find its own way forwards, build itself up, and it does, all the way to the heavens themselves. Twenty five thousand years of growth and prosperity, the colonisation of the galaxy, humanity's rising star.
And then the Age of Strife hits. Old Night, the gestation of Slaanesh. Worlds are cut off, psykers spring up like weeds, artificial intelligences go haywire, the whole thing unravels.
So then this immortal takes on the mantle of The Emperor, fashions himself a radiant golden form, and steps in. Because if he remained passive, there wouldn't be a human race left. He wrests control of Terra, unifies the world and breaks down the ideologies that inevitably throw up barriers between subgroups. Religions, kingdoms, nations, it all gets scrapped in the name of Unification.
He creates the Space Marines, because if he's going to actually lead armies and rule the galaxy, he wants soldiers that a) can keep up to his standards somewhat, and b) won't die of old age before he's even gotten started. To lead these exceptional soldiers, he needs exceptional generals, and this might just be the chance for him to have company he might be able to relate to, even slightly. So the Primarch Project kicks off. The Emperor looks into the Warp and makes bargains, looking for power to infuse his 'sons' with, to bring them to a level close to his own.
It goes pretty well. There's a slight hitch, and the Primarchs get scattered, but that's not too bad, and the Emperor can actually use it as a good catalyst for the whole Great Crusade thing. For two hundred years he leads the Crusade personally, finding his lost Primarchs and rebuilding humanity's empire amongst the stars. He's distant, as his sons see it, both because of the aforementioned difficulties an all-powerful immortal has in forming relationships with lesser beings, and because he's got a thousand things to do at once. Twenty sons to shepherd, two of which he has to (probably) kill and two others who he's constantly having to keep an eye on in case they need to go too. Thousands of fleets to manage. A million worlds to watch over. A court full of intrigue back home. Politics, shenanigans and power plays from greedy mortals who'll only be dead in a few years anyway. A hundred actual battlefields that he has to personally step onto and kill shit on a regular basis. The warp's hanging over everything, and even though he's done his best to make an empire that'll have the best chance of avoiding falling into its grasp by accident - a faithless society that scorns gods - he knows he'll have to do something, even if only because some of the Primarchs are getting curious and in over their heads.
So he decides to build a webway. A human webway, alongside and within the ancient Eldar webway. That would remove the reliance on the bloated and inbred Navigator Houses, remove the need to stare into the warp, and connect the worlds of the Imperium in a way that can resist another Old Night.
To do that, he needs to take some time off. Not long, just a couple of decades, to get the fiddly bits right. He can't just brute force it, not if he wants it to work without him. So he puts his favourite son, Horus, in the place of Warmaster, tells him to keep up the good work, but daddy's got important work to be doing. He doesn't tell anyone *what* important work, because, well, he's not a social genius and he honestly just thinks they don't need to know. The things they *do* know are getting them in enough trouble as it is. He spent thirty thousand years unlocking the secrets of the cosmos, they can wait a decade or two for Christ's sake.
So he's back in the palace, probably losing track of time, and doesn't want to be disturbed.
Then, practically as soon as he's started (from his point of view), Magnus smashes the whole project to shit talking some shit about Horus being a traitor to the Imperium, scares the peasantry shitless manifesting as a giant blazing crimson demon, and vanishes. The Emperor tells Leman Russ to go fetch Magnus so they can have a nice, long talk about knocking before entering.
And before he's even gotten started on picking the pieces back up and clearing out the flood of daemons that Magnus has let into Terra, the news comes in from all over - Horus is actually a traitor to the Imperium and has started a fucking civil war.
The Emperor, at this point, is likely thinking "What the fuck?" He's left them alone for a few fucking years, and it all goes completely to shit. Leaving them alone so he can do other things than lead an army was the entire fucking point of creating the Primarchs in the first place.
God dammit people.