They killed billions of Forerunners and all 30 of his children. They took the Mantle of Responsibility into their own hands when dealing with the Flood, which the Didact feels lead to the downfall of the Ecumene. At the twilight of his civilization, he was pushed to take drastic actions to try and preserve it, leading to him justifying the digital enslavement of humanity and any other species that would stand up to the Forerunners.