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Angron
The red angel. Lord of the Red Sands. The Broken Primarch
From the moment of your creation you were doomed. How terrible it must have been for you be barely have been alive and already attacked by warriors of an alien race. Their leaders had seen into the future and knew what you would become. It's interesting to consider if they truly believed they could prevent it or where they simply playing the part destiny had given them. You defeated them but it would have been better if they had killed you.
You survived but were found and enslaved. Your skull ripped open and a horrible tool hammered inside that drove your very potential into one purpose: rage. You were made into a tool but in your finest hour you fought against that. You fought and fought until you were on a mountainside facing death. You would have died but you would have died free alongside your brothers and sisters.
No one could blame you for your reaction to what happened afterword.
Your true father came and took you away leaving the only family you knew to die.
You were right to be angry. You had been taken away to the stars when you had no wish to do so, given command of an army you didn't desire in the slightest, and told to bring other worlds under the Imperium's heel.
What you can be blamed for is what happened next.
You rightfully hated the Butcher's Nails, seeing them as a tool of monsters yet you barely hesitated to force your legion to do so. They went along in the hopes of gaining their gene-sire's affection yet you treated them with nothing but contempt. The World Eaters were renowned for their fierce loyalty to each other yet you destroyed that making them renowned for their bloodlust, butchery and insanity.
But do you know what is truly disappointing about you?
When Mankind's first empire countless worlds once peaceful and safe became the opposite. Humans were enslaved, tortured, murdered and lost much of their honor and were reduced to little things simply trying to survive in a harsh and uncaring universe.
You could have understood better than any of your brothers and your father himself what is meant to be weak, to have no control over any aspect of your life, to be a slave. You could have had more empathy for mankind than any other primarch and the Emperor himself.
That is the truly disheartening fact Angron.
You could have been more.
You could have been Mankind's greatest Hero instead of one of its worst villains.