That's enough responses! I will speak!
For a brief overview, the Emperor of mankind is the product of Humanity's psychic awakening into the universe. Shamans, Soothsayers, and the primitive Psyker all had a vision of what will come to pass in the future, and were given a vision if that they all took their lives at that moment, they would be reborn into a being that could save humanity.
They did, and it did come to pass that all their psyker might was reborn into the Emperor of Mankind. A being that had few equals in all of the Galaxy. The time he was reborn is questionable, but it is said that he lived with humanity for millennia, reflecting on the weakness of mankind. All the while reflecting into this parallel version of reality called the Immaterium, also known as the Warp. He knew while humans warred with each other, there were far greater dangers both in Real Space (or the Materium) in untold Xeno Hordes aand being grown in the warp by humans' hopes, passions, and beliefs.
Because each of those very human aspects take form in the Immaterium. With enough pooled energy, they might give birth to lesser dameons. The shared belief could grow it more into a greater daemon, or even a God of the Warp.
And that's where I think the Emperor is the cause of all of humans' downfall.
The Emperor always knew the dangers the Warp contained for humanity. And how it functioned. He knew religion and faith created actual daemons that would feast on the souls of the faithful after their passing. There is no heaven in 40k. There is only the warp, where all souls go when they die (unless you have Aeldari soulstones or belief in the Laughing God who will shepherd your soul).
And what does the Emperor do with this knowledge?
He uses his Psyker Might that can go onpar with the 4 Chaos Gods (the most powerful beings in the Immaterium) to project golden radiance from himself. He storms from the front, a 14 foot Mountain of a Man, with his genetically enhanced 7-8 foot tall Astartes (Space Marines) and his 10-12 foot demi-god artificially made sons called the Primarchs. He talks through entire populaces with his mind. He throws Psyker mind around, dispatching every foe and plague that has held countless human worlds in peril for centuries. Then he promises to come to the aid of the planet as long as the planet remains loyal to the Emperor. And Oh yeah, not to believe in anything but science. Nevermind his magic. Ok, Byeeeeee!
And what's the result? After the Horus Heresy, the Entire Human Population looks at him like a God. He is called "The God Emperor of Mankind".
... what the hell was he expecting?
If he didn't want to seem like a God, why did he do everything POSSIBLE to look like a God? He can control his radiance and even his height. He doesn't have to appear 14 foot tall. There's question if he is actually 14 foot tall to begin with, but he could make himself appear to be much like a normal human if he wanted. He could have engineered his Astartes to be human size and say that they were regular humans in kick ass Iron Man suits. He could have hid his psyker might and claim it was advanced weaponry.
Or, better yet... HE COULD NOT HAVE SHOWN UP AT ALL!!! People will start to have questions when a 14 foot magic man that doesn't die, who your great great grandmother told you stories about years past, took a selfie with him... and so did her daughter, and her daughter, and so did your mother, and zog it, so did you. A God is more of a sensible answer than "Oh, no. He's just really a special guy".
Erebus wouldn't have been a thing if they were just given the lie that they were constantly on really great steroids, and that the mechanical suits they were in conjunction with chemicals keeps them young for much longer than the average person. There were explanations to be had! But he chose to just do everything possible to look like a Divine Being, but say "I'm totally not bro".
It is said the Warp grew to almost uncontrollable levels during the 10,000 years of the Emperor being placed ont he throne. And that's entirely his doing. He made every choice to undermine himself and his efforts. And warp swells with delight over that fact.