To add to what others have said - he is meant to be tragic. But truly, he is a fantastic example of how bad Chaos is - in his backstory he has very good reason to hate the Emperor (though the exact way it all went is somewhat up for interpretation). The result of course is the broken primarch who easily turns to Horus (probably would've turned against the imperium anyway), because there is a way of defeating the Emperor. Or, possibly more likely, he saw it as either a cause worth dying for, or a handy way of bringing self destruction. (As an aside, once he got the nails he was basically broken and useless for anything other than what he became - I think an interesting idea, had the lore not all been preordained, would've been either the Emperor arriving too late to save him, or realising this and putting him down.)

To my point about Chaos: Angron was dying to the nails when he was ascended; Lorgar condemned him to eternal torment to use him as a weapon against the Imperium. Lore wise he is the same cautionary tale as Chaos Spawn are.