Quote Originally Posted by Mordokai View Post
As the story goes, the Emperor held himself back in the fight with Horus because he had trouble fighting his favourite son. However, when Guilliman is resurrected...
Pfft. It's directly stated in Master of Mankind. No need to wait for G-man to walk again. Malcador says it, straight up.

We learn from Roboute that Emperor always considered primarchs more as tools.
We learn it from The Emperor himself.
We learn it from Dorn, who tried to earn his Father's love, who ultimately had to accept that he would never get it.

So if we take that story at face value... why did Emperor held himself back?
My guess is two-fold:
1. The Emperor's heart grew three sizes that day, or
2. Horus really was his favourite.