In your personal opinion what was each Primarch's, not just the Traitors, greatest flaw?
OK, let’s take it from the top!
I. Lion El’Jonson: Paranoia. His inability to trust people is the reason it took Luther to actually get his crusade against the Great Beasts going, and gives him an independent streak that causes major problems in Imperium Secundus.
III. Fulgrim: Pride, vanity.
IV. Perturabo: There’s anger and paranoia, but bitterness is the core. He appears to deal with things stoically, but underneath he builds up resentment until he snaps - the anger is the release, the paranoia the defence mechanism to lower his expectations of others.
V. Jaghatai Khan: Unpredictable and impetuous, he lives in the present and doesn’t do much in the way of planning ahead.
VI. Leman Russ: Raised a legion of hypocrites in spite of his own surprising self-awareness. In fact he was probably too self-aware, and got swept up in paying the part he believed he was meant to play.
VII. Rogal Dorn: Uncompromising and inflexible, which stems from his idealism. He attempted to fortify the palace effectively with one arm tied behind his back because he wanted to make sure everything could be put back. Incapable of bending, he either stands strong, or breaks, which nearly kicked off a second civil war.
VIII. Night Haunter: A total psychopath utterly convinced his actions were justified. Very much a “two wrongs make a right” kind of guy.
IX. Sanguinius: Dare I say a bad word about Sanguinius? Well, yeah. There’s obviously some anger issues in there somewhere, hence the black rage, but he also has a case of imposter syndrome and a streak of self-pity. He’s terrified of letting other people down and overly concerned with his own flaws.
X. Ferrus Manus: Anger, which his sons are so eager to overcome that they virtually lobotomise themselves.
XII. Angronius Thal’kr: Anger would be too easy, although it does go beyond the Butcher’s Nails. I think spite may be closer to the mark. He doesn’t believe in anything, he defines himself by what he is against.
XIII. Roboute Guilliman: Views everything as part of the bigger picture. Individuals become part of systems to which they must conform, yet the system is ever-changing as he steals his brothers’ ideas. Struggles with deviations from expected norms. His focus on what was best for the Imperium as a whole after the Heresy contributed to him handling Dorn very poorly.
XIV. Mortarion: Insecurity, which he projects onto others in his loathing of psykers after he was too weak to stop his father, and eventually turns him into everything he hated most.
XV. Magnus: Arrogance, and a surprising impetuousness and naïveté that flow from that. Believing he knows best, he doesn’t think about things as much as he should, meaning he misses things that should be obvious to someone as intelligent as he is. The galaxy’s dumbest smart guy after the Emperor himself.
XVI. Horus Lupercal: Absolutely desperate for attention.
XVII. Lorgar Aurelian: Very naïve and trusting, which results in him putting people on a pedestal thank frankly don’t deserve to be there. He somehow managed to overlook all of Kor Phaeron’s many, many flaws (aka his personality) for several centuries.
XVIII. Vulkan: Relies only on himself. The first thing he does when he finds out about Horus is order a giant weapon stash to be destroyed, trusting no-one else except himself to use them. You have to wonder how much he really thinks about what he does given that he also managed to feel bad about killing an Eldar kid even though he’s totally on board with his Father’s planned genocide of their (and every other xenos) species.
XIX. Corvus Corax: Self-destructive inasmuch as he is prepared to do bad things for the right reasons, but can’t quite get over having done them. He believes he’s doing the right thing even as the doubts fester, and the end result is self-loathing.
XX. Alpharius Omegon: A stealth specialist who feels the need for everyone to know how good at stealth he is.