So briefly; Bobby G as we all know is a master planner and architect, just like a certain blue feathery god of deception. After the big E discovered the word bearers were worshipping him he gets mad, and sends his perfect and loyal ultramarines of Guilliman, an image he almost certainly cultivated with intentional care, to blow up some of the word bearer’s shit. As we all know the word bearers run away, and fall into the grasp of chaos. As we all know the heresy goes into full swing, and later Horus is defeated with the emperor placed on the golden throne. Here we can see the ultra marines primarch pulling his first power move, the codex astartes, which confidently leaves him with the most power in the imperium. It feels like some grand scheme, prefect for some nerdy architect. What his end goals are now with sole control of the imperium, nobody can say.