How do?
Nice and straight forward this one, as it's about your favourite Traitor Primarch, and the reasons behind that favour.
My one is easy.
It's Perturabo.
One of the Emperor's two siege masters, he got dealt an exceptionally ****ty hand. Whereas as Dorn and his Imperial Fists got all the breaks and the glory, the Iron Warriors got stiffed with the grubby, grotty hard work of laying siege, and quite often weren't even given the honour of the kill, as other Legionnes Astartes got to exploit the breaches bought with Iron Warrior blood.
I mean, wouldn't you be pissed off with your Dad if you did all the work, and your arse of a Brother repeatedly got the praise? And I know he's not the only one to get the ****ty end of the stick, but he had potential.
Angron? Stiffed of a glorious death, but ultimately guided too much by the brain butchery in his head.
Lorgar? Victim of a necessary censure, and really, really bad advisors. In a sense, both Lorgar's fathers stiffed him.
Horus and Fulgrim don't really count, as their turning wasn't particularly self determined.
Alpharius? Listened to Xenos. Took their word for it. Prick.
Mortarion? Yeah ok, similar situation to Perturabo.
Magnus? Betrayed by Horus, not The Emperor overall. It was Horus that set the Wolves on him!
Rights, over to you. Let's hear some sympathy for the devil!