Or "I'll capitalize on the predictable actions of my enemy and engineer those actions into my own plans.""I'll deliberately allow a massive civil war to start between my sons, a war that the traitorous ones start out with a significant advantage in and retain throughout most of the war, with the planned outcome to be that I just barely manage to eck out a win on the narrowest magin possible" is significantly more stupid than "Yes, angeron is a mad dog and a threat to everyone around him, but surely if I keep him pointed in the direction of the enemy it'll be fine".
The Emperor set up an Imperium that absolutely was going to have a massive, destructive, civil war. The masses of humanity were not going to stand for the rule of the Primarchs and their Astartes going forward, transhuman rule was indeed explicitly rejected by the Emperor and directly opposed to his publicly stated premise for the entire Crusade. The Treaty of Olympus Mons solved none of the actual issues between the Imperium and Mars, and once the galaxy was secured either the Legions would be turned against the Mechanicum to eradicate the Cult Mechanicus and return actual science and study to the masses or the Mechanicum would act against the Emperor for his constant violations of his agreement with them; Mars was also ripe for a civil war of its own. Lorgar remained unbowed, Magnus was violating the Edict of Nikaea literally within hours of it being articulated, Angron was still Angron, the Emperor's Children were still a degenerate pleasure cult lead by a daemon possessed Primarch; any, all, or a combination of these issues is going to come to a head and in every case it will be a civil war.
All of that totally ignores that the Imperium as structured is the most pointless government ever devised. For a polity nominally driven to secure humanities future and unify it under the Imperial Truth to restore what it lost, it had a government utterly focused on persecuting a galaxy spanning war. And even then, it was fighting said galaxy spanning war without ever bothering to even have a logistics core, a developed chain of command, or standardization of any kind.
Sure, if the Heresy didn't occur or the Emperor remained active after it was over then reforming the system of government into something that functioned wouldn't have been beyond him; but it would have been easier to build it in from day one and the Emperor never did. There is a reason that Guilliman had to basically restructure the whole system from scratch post Heresy.
Again, either the Emperor was fundamentally a retard (which is the diametric opposite of every statement everyone ever made about him in any of the Lore) or he was not undertaking the Great Crusade for the purpose of accomplishing any of his stated goals.The Hersey as portrayed in the GW novels is incredibly stupid, because the project was badly mismanaged from the start (as shown by them choosing to start the series with Horus's fall), and was probably beyond GW's ability to handle anyway. There are worthwhile bits here and there, but frankly the default attitude toward sny derived from it should be "ok, so explain why I should accept this bit of fluff here as canon". You certainly shouldn't just run with all of it as canon and the lean into the stupidest fan theory ever to worm its way into offical material.
I mean take Magnus. Everyone and their mother knew he was going to ignore the Edict, obeying it is fundamentally at odds with his entire character. What is the Emperor's big project? The Imperial Webway, a massive undertaking of psy-arcana. Who is supposed to sit their ass on the Golden Throne and control said Webway? A red cyclops. I know, why not bring Magnus back to Earth to help with this project while sending some spies to keep an eye on his legion.
Or the Imperial Truth vs. religion. Despite literally all of human history, which I have personally lived through, making it absolutely plain that human beings have an almost obsessive need to believe in religion of one description or another, especially in less than stellar circumstances, I shall push militant atheism onto the whole of humanity. And I shall push this atheism while playing into every bit of religious iconography that I can find, and while actively, personally, doing everything that I can to appear divine.
Or the "You shall accept all of my actions and statements at face value, despite the fact that basically every one of those actions is shrouded in secrecy, utilizes deception as a matter of course, and involves lying through me teeth to everyone and their mother." I mean literally ANYTHING that presupposes that the Emperor cares a solitary damn for honor, morality, honesty, integrity, or sacrificing whatever lives he needs to accomplish his ends (and at any scale) is in fundamental conflict with literally everything we are ever shown about the Emperor's character, actions, and beliefs.
So why should we accept that he was honest in his public statements concerning the purposes of the Great Crusade and forming the Imperium?