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. 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?