I may be missing so details so I'll say this first. From my understanding the Emperor arrived for Angron and Mortarion during both of their rebellions. Both Angron and Mortarion's rebellions would have failed. Mortarion's gas masks weren't strong enough, and Angron was outmatched and couldn't think straight due to the nails. My question is why did the Emperor step in and save Mortarion's rebellion but just snatch Angron away from his? I imagine it was WELL within the Emperors power to save Angron's rebellion, so why didn't he? Isn't Angrons biggest hate towards the Imperium that they didn't save his gladiator family? Just this small action I feel could of had MASSIVE implications for the story. Who knows, if Angron liked the Emperor more for saving his family they might have even let him try to remove the nails (that's a big stretch tho)