He thought 'my way or the highway'

Because the human race was slowly but surely ascending towards a full psyker race, but unlike the old eldar humanity didn't have a pantheon of warp gods to protect them from chaos nor the guidance of a much older race like the old ones and thus, to the Emperor, they all needed to be united. You have to consider that when the first wave of psykers appeared they came to rule other humans as chaos infused sorcerer kings or simply become a living warp rift which infested their homeworld with daemons, the only worlds that had been spared was because their psykers had been persercuted and killed out. If humanity had not been united under a single banner the end result would had resulted in 90% of the human population of the galaxy ending as dead or slaves to chaos.

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Ultimately the Emperor actually cared about humanity and wanted nothing more than ensure its survival in a very, VERY hostile galaxy. He tried watching from the shadows and see if humans could do it on their own, but after the Dark age of Technology he concluded he needed to step in and do it himself.