Horus

Horus was the main figure in the event known as the Horus Heresy (this is NOT a dead giveaway. Oh no) in which he, yeah you guessed it, betrays his father of sorts - the Emperor and turns to Chaos... heretically. Before that he was the Primarch most favored by the Em--oh come on! If you really need to be told any of this, what the fuck are you doing on /tg/ in the first place?

Long story short, the Imperium got fucked up because this guy had daddy issues.

Horus was weak, Horus was a fool. He had the galaxy in the palm of his hand, and let it slip away. - Abaddon.

Like you're one to talk, Failbaddon. Remember: 13 failures.