[quote]So fades the harvest of my betrayal[/quote] He was betrayed by the Librarian because he wanted to compose the entire human race to fight the Flood, she betrayed him and placed him in the Cryptum on Requiem. [quote]Even these beasts recognize what you were oblivious to human[/quote] Didact thinks that the Forerunners are the worthy inheritor of the Precursors mantle, not humanity, thus making them the species that must rule the galaxy. [quote] The Librarian left little to chance didn't she? Turning my own guardians, my own world, against me[/quote] Requiem was essentially the main shield world from which the Didact waged war with the Flood. It was his option to fight the Flood instead of the Halo Array. His Prometheans were turned by the Librarian in order to protect the place from anyone trying to access the Didact. [quote]But what hubris to believe she could protect her pets from me forever[/quote] The Librarian took special interest in humanity and devised plans to guide us after being de-evolved after the Ancient Human - Forerunner War. It's most likely that it's because we managed to defeat the Flood, and the it is said the Flood is challenge set by the Precursors to see if that race is worthy of the mantle. Humanity defeated the Flood, the Forerunners did not. [quote] If you haven't mastered these primitives, then man has not obtained the mantle. Your ascendance may yet be prevented[/quote] The mantle is a mythical right from the Precursors over who should be the galaxy's guardian and protector. Basically be the higher race that should guide all others to advancement and such. The Didact takes this as a reason to be superior and dominate the rest, even if the mantle is not corresponded to the Forerunners. [quote]Time was your ally, but now it has abandoned you. The Foreunners have returned. This tomb, is now yours[/quote] The time humanity had was given thanks to the Librarians betrayal of the Didact. Now that he is out again, he is out to compose the human race in order to fight the Flood, which is bound to be returning to the galaxy from dark space, as it did with ancient humanity. Take note that the Flood outbreaks we know have come from Forerunner installations in quarantine. Flood coming from outside the galaxy is thousands of times more dangerous and in even bigger numbers. Hope that helps. It's all in the novels. Particularly the Foreunner trilogy.

Watch the cutscene in the middle of Reclaimer and the Terminals, bro. Everything's explained pretty clearly. The Ur-Didact took the Composer and committed crimes against the Mantle by using it on the Librarian's humans on Omega Halo, she pursues him and incapacitates him aboard his ship with a Binary Rifle. She then seals him away inside his Cryptum in Requiem so his mind can recover through many years of meditation on his choices so that when he awakens he will find a newly evolved humanity and he will help them attain the Mantle. However, Ur-Didact's experience in a Flood-infested system and torture at the hands of the Gravemind drove him into insanity, and the Domain which was supposed to heal his mind in the Cryptum was destroyed when the Halos fired, as the Precursors reveal that the Domain, Mantle and Organon (a Forerunner myth) are the same and based off the Precursors' neural physics which the Halos destroy. Ur-Didact is left to stew in his hatred and Flood-addled insanity for 100,000 years. He awakens rather grumpily when he sees that humanity, the enemy he fought against and the cause of the death of all his children, has endured and are taking what he believes belongs to the Forerunners alone.