Well first, when he was kid, he killed another kid and took his face and name. Then, after becoming a space marine, he started corrupting his Primarch, Lorgar, into joining Chaos. After that, he then poisoned the Primarch, Horus, with a Chaos infected dagger to make him join Chaos.
When Horus fell to Chaos, all things went to poop and after Horus mortally wounded the Emperor (not before being obliterated out of existence), he and the rest of the traiter legions went to the Eye of Terror and still forms hundreds of Chaos cults to this day.
Caption this Meme
