The emperor was formed by the sacrifice of mortal psykers with the collective goal of creating a shepherd for mankind. His sole reason for existence has always been to protect and nurture mankind.
Not actually fact. It's plausible. But the reading I've done says the Emperor was created to defeat Chaos, first and foremost. He took on the role of mankind's protector but that wasn't what he was created for.
Also "nurtue" is perhaps too strong. He certainly wanted to unite humanity...using bolter rounds. You can't really call the Great Crusade a war of nurturing. The Emperor decimated million of human lives in the name of unity, and pretty much as soon as he felt the crusades were over, he left those worlds in the hands of a growing bureaucratic class and holed up on Terra. The Imperial Truth is seemingly beneficent when compared to full scope of horror in 40k....but from the perspective of those who opted for "compliance" over "annihilation" it probably didn't seem that way. The primary purpose of the Imperial Truth was to create a secular universe, to defeat Chaos. The Emperor was fully aware of the value of non-secularism. He just believed, in an absolutist fashion, that starving Chaos of belief was more important.
If we accept that the Emperor's primary mission was to defeat or at least stymie Chaos, then making sure humanity didn't succumb to it as a whole is a necessary part of that equation. The Emperor is usually portrayed as being selfless in his defense of humanity, its biggest fan, but considering how fucking ruthless he and his Imperium became toward humanity, even during the Great Crusade, I think it's less selflessness and more cold, hard pragmatism. Humanity, to the Emperor, the Primarchs and the Legions, are a playing piece on a chessboard. The "father of humanity" and all that is propaganda at best, and rationalization for all the misery of his Imperium, at worst.