Perturabo valued skill over class and wanted to instead convinced worlds to join the Imperium through diplomacy. He also wanted the people to choose their leaders instead of the leaders being chosen for them.

However when Perturabo joined the a Imperium he was instead given the task of sieging. the enemy, there was no glory for him, no way to achieve his dreams of utopia, only long grueling warfare. Perturabo kept doing it because he believed that if he kept doing the unwanted but necessary tasks then one day someone might see all he sacrificed and finally recognize his genius and philosophy.

Perturabo had many flaws don’t get me wrong. He was born already with an innate knowledge of the universe and was adopted by a tyrant who used him essentially as a “trophy kid”. Perturabo never had to go through the normal process of growing up, he never had to make mistakes and learn from them. His mood would swing wildly so he would bury his emotions under a facade of cold logical facts and numbers. To top it all off he was able to see the Eye of Terror and felt it constantly watching him wherever he was.

So in the end you have Perturabo. A child genius who was never able to grow up. A philosopher who was always treated like a janitor, an angry and bitter person who suppressed his emotions. A person who lived his life always being the best on his homeplanet who then joined the Imperium and was under appreciated compared to his brothers. He was like a pair of sculptors tools that had been reforged into nothing but a blade.

Eventually the final straw was when his homeworld rebelled, Perturabo followed what the Imperium always did and subjugated his homeworld. The massacre that followed broke him emotionally when he realized what he had done. At the end of it he was nearly catatonic, asking himself over and over “what have I done?” While prayer against hope that some of his legion had refused his orders. Perturabo eventually joined Horus because at least Horus seemingly recognized Perturabo’s value and even agreed to pardon Perturabo for the massacre of his homeworld (keep in mind that Perturabo was essentially suffering from PTSD at this point from what he had done). That’s when Perturabo really fucked up and soon became the only “sane” man among the forces of chaos.