In "Dawn of Fire: The Silent King", it's revealed that Roboute is taking promising Primaris marines under his wing and grooming them to be more than warriors. He is molding them to be leaders of men. Marine rulers and governors over mankind meant to lead the species to peaceful times. His old dream of Marines ruling and guiding humanity never died.
That would explain a troubling pattern with Roboute. He keeps sidelining baseline men leaders in favor of marine rulers. He forcefully reunited the 500 worlds of Ultramar and revived the Tetrarch Council, whose members are high-ranking Marines chosen by Roboute himself to rule Ultramar and its hundreds of worlds.
He bestowed on Dante of the Blood Angels the title of Warden of Imperium Nihilus, making him the effective ruler of half of the Imperium.
He named Captain Vitrian Messinius the Warden of Nephilim Sector, granting full authority over the Imperial forces and worlds within that shrouded part of the Imperium.
The template Roboute intends to reform the Imperium into is Ultramar. And who rules Ultramar? The Space Marines!
Again and again, Roboute is imposing Marine rule over mankind. This is explicitly against the Emperor's wishes, according to Malcador. The Emperor did not want transhumans to rule over humanity.
