Ultimate Job Security: Most of the traitor Primarchs were already unstable tyrants, criminals and madmen when the Emperor found them, but he still put them in charge of personal armies of super powered space soldiers.

However, a combination of personal doubts, bad luck, strained relationships with their brothers and father, horrible circumstances, and manipulations from the Ruinous Powers made them turn on and destroy their life's work. At one point, while they had their own ways and ideas of doing it, they legitimately wanted the best for the Imperium. Tragic Villain: All of them, though some moreso than others.Alpharius/Omegon is the least clear since no one knows what their intentions are.The Emperor humiliating Lorgar, destroying Monarchia and shattering his beliefs.Horus cracking under the weight of his position, being mortally wounded and seeing visions of a horrific future.Magnus learning to project himself into the Warp and making contact with Tzeentch.Mortarion having his victory over his father 'stolen' by the Emperor.Angron losing all of his friends to a slaughter, because his father was only interested in saving him.Konrad Curze destroying his own planet after its rebellion convinced him he wasted his time bringing it peace.Perturabo's legion having to put down a homeworld rebellion after yet another hellish, thankless siege.Fulgrim taking up the daemon-possessed Silver Blade.

Possibly subverted as of Wrath of Magnus, with it being noted that several Daemonic Primarchs have been seen moving to Cadia.

Magnus took part in a direct assault on the Space Wolves' homeworld but was defeated and hasn't left the Immaterium since.

Orcus on His Throne: Only Angron has ever left his daemon world to war with the Imperium in a large scale.

Several of them, like Magnus ◊, still prefer to appear in a close approximation to their original forms, but for others it's not an option or they don't care.

One-Winged Angel: Their transformations into Daemon Princes have changed them in monstrous ways, with Angron and Fulgrim being the best cases (Angron now resembles a Bloodthirster and Fulgrim is a four-armed snake-man).

A lack of self-awareness combined with an inability to take responsibility for their actions is the cause for most of these Primarchs turning traitor.

Never My Fault: Remove this character flaw and most of the Primarchs never turn agains the Emperor.

A combination of the savagery of the Crusade, bitterness towards the Emperor, personal trauma, and the corruption of Chaos warped all of their originally noble goals into something unrecognizable.

Motive Decay: All of them started out with the intention to improve the world around them in some way.

Freudian Excuse: Many of the Chaos Primarchs have these to justify their betrayal.

Now their names are spit upon by those who still remember them and they seek to destroy the empire they once served. Fallen Hero: Each of them was once one of the Imperium's greatest warriors and leaders.Demon of Human Origin: The surviving Traitor Primarchs have ascended to Daemon Prince status.Some of these were legitimate (Lorgar's faith in his father being shattered due to the Emperor's destruction of the crown jewel of his work and falling prey to Chaos in his moment of weakness Angron losing all of his friends and comrades when the Emperor refused to help them take their world and only saved Angron when he could save Angron's army as well) while others were blown out of proportion or misled completely (Horus tricked into believing the Emperor intended to rule humanity as a god Magnus being played like a fiddle by Tzeentch). Calling the Old Man Out: Most of the Traitor Primarchs turned due to actions that the Emperor or their loyalist brothers did to them that they never forgave.Broken Ace: While most of the Primarchs were this, the traitors took it up a notch, as it was their very nature as broken beings that led to their corruption.

Horus heresy tv tropes