Let me take a stab at it.
I think the crux of the thing was pride. Horus was the Emperor's favorite son, chosen Warmaster of his armies. Horus led the emperors armies across the stars concuring in his name. After a while Horus started getting kinda pissed off that he was basically doing all the hard work while the Emperor stayed on Terra and claimed the glory.
At some point Horus fell to a poison created specifically to kill Primarchs (Nurgle) which eventually overcame his system. In a desperate attempt to revive their fallen Primarch the 'Sons of Horus', later re-named the Black Legion I believe, took him to a shrine world and locked him in a serpent temple of some kind. One inside the forces of the Old Gods corrupted and revived him. A servant of Chaos (one of the first traitor marines) showed him a portion of the Future where the Emperor is the GOD-Emperor and Horus and certain other Primarchs are not represented (because they turned to to Chaos, ironic huh) and he makes his own conclusions. You can sorta see what happens after that.
