Tens of thousands of years before the Forerunner-Flood War, Ancient Humanity lived on the outskirts of the galaxy.
Ancient Humanity began to aggressively push into Forerunner territory, provoking the Didact and his Warrior-Servants to stop their advance. Throughout the war, the Didact lost all of his children and millions of warriors. The Humans wiped out planets (using what appears to be a form of glassing) and fought hard for hundreds of years.
The Humans were actually retreating from the Flood at the same time, and were destroying Forerunner planets to stop its spread, but they didn’t tell the Forerunners because they wouldn’t stop it fast enough. So, to the Forerunners, this was all just sudden aggression.
Over time, the Forerunners encountered the Flood, and the Didact blamed the humans for leading it to Forerunner space. The Humans had allegedly created a cure for the Flood, but in the last years of the war, they destroyed it so the Forerunners couldn’t have it.
So, Humans aggressively pushed into Forerunner space, resulting in the death of the Didact’s children and millions of fellow warriors, and in a way, lead the Flood to the Forerunners.
Fast forward a few thousand years, as the Forerunners prepare for the Flood’s return. The Didact is exiled for opposing the Halo rings, awakening a thousand years later by a young Forerunner and two humans in the middle of the war with the Flood. He tolerates these humans now, but is eventually captured by a Gravemind and tortured. This event drives the Didact insane, and the Forerunners’ incompetence in fighting the war so far is frustrating.
He seeks several different ways to stop the Flood, including mutations on himself, which fail and destablize his mental health even further. The Composer is his last solution, an army of mechanical warriors who can’t be infected.
The Didact uses the Composer on his own Promethean soldiers, and initial results are promising. So he seeks more.
Viewing Humanity as still having the potential to be a threat in the future, and insanity clouding his judgement, he composes the humans at the Greater Ark and flees to Requiem with the Librarian close behind. The Librarian, his wife, ends up shooting him, locking him in a Cryotum. She hopes the Domain will help heal and clean his mins, but the Domain is destroyed by the Halos.
So… he sits there for 100,000 years wallowing in his own insanity, blaming the humans for everything that’s happened.
The Didact’s motives are a mix of grudges from a previous war, awakened and pushed out of control by insanity, and frustration for not being able to save the Forerunners.
When he awakens after 100,000 years of meditating on insanity, he sees humans rising. His old grudges resurface and he seeks to finish what he started, removing Humanity as a threat and securing a large force he can use to protect the galaxy and secure the Forerunners’ dominance… a notion the other Forerunners don’t agree with.
The Didact’s story is a very tragic story, creating one of the most compelling and interesting characters in Halo.