That's mainly because the guy is just called Horus and his betrayal is called "The Horus Heresy"
That shit looks so awesome. I really want that dark and brutal shit as a movie trilogy
The lore about the emperor is not clear and intended to be confusing/diffuse. Some say one interpretation of the Horus heresy is to erase whole marine chapters, just like the storm warriors. They were not useful anymore. And we just don't see the greater plan. Or the emperor is just flawed like any human greeding for power.
It's this a game or a movie?
That's THE Horus Heresy to you, Greenskin!
Just like the emperor said: "You beat me Horus, you really are the Warhammer 40k."
I know it's supposed to be a meme or whatever but his name is horus lupercal. Are you trying to disrespect the warmaster?