A decent idea that has been thoroughly character-assassinated by the atrocious Horus Heresy series, a collection of novels far more interested in one-upping each other by presenting new shock twists and determined to give the author's favorite characters the most legit manpain, rather than presenting a coherent storyline.
The Emperor is at his best in the old Index Astartes articles, wherein he is presented as a well-meaning but deeply flawed figure who wants to advance humanity's existence and for much of his own life has worked behind the scenes since his birth in 8000 BC, the spiritual amalgamation of many thousands of psykers ("shaman") who foresaw the coming of Chaos and scarified themselves to find a way to preserve humanity, ultimately coming to the fore following the Age of Strife in a determined effort to unite the disparate elements of our race before we are snuffed from existence by war with other species or by the ravages of the Warp. Despite being very good at the big-picture view, he has faults and foibles as does any human being and, in the fashion of classic Greek tragedy, he is ultimately undone by them. His love for his favorite gene-son Horus blinds him to said son's increasing vanity and ambition, and similar mistakes made amongst the other primarchs (best shown in the stories of Magnus and Mortarion, IMO) ultimately leads to the downfall of the Imperium (itself yet a deeply flawed, incomplete project at the time of the Heresy) and the sundering of everything he worked to create, twisting it into a hellish dystopia that grinds humans by the millions beneath its gears with each passing day.
The Emperor is at his worst in the above-mentioned Heresy novels, in which he is a floundering buffoon who couldn't organize a surprise birthday party let alone an empire, petty and malicious in spirit on the order of several MegaHitlers as he destroys people and things the instant they aren't of use to him and at times seems to evince none of the positive human emotions, utterly incapable of connecting with others on even a basic level. Oh yes and literally everyone with an ounce of experience knows better than him about how Chaos works and that the Imperium will fall to Chaos and tries to correct his stupid idiot mistakes but he's too stupid and idiotic to listen because he's the worst and also why didn't you love me dad. Also his backstory is retconned into being a dude who made a bargain with Chaos and promised to deliver humanity to them in exchange for the knowledge to stuff a bunch of warp power into human jackets he could call primarchs, then went back on his word because apparently the Chaos gods are such flaming morons they handed this guy a truckload of power based on a pinky swear with no take-backsies.