Let's just say that after reading Silentium, there is a pretty damn good reason that the Didact is the way he is in Halo 4.
Major Silentium spoilers:
The Didact in Halo 4 is the Ur-Didact, the original. Now, the Librarian maintained that the Bornsteller/IsoDidact is her real husband, despite him being the one that the original Ur-Didact imprinted onto the younger Forerunner, since the Ur-Didact thought he was going to die.
The IsoDidact then went about completing the plans of the Didact as his imprint grew stronger. The Ur-Didact ended up surviving, and his memories of conversations with the Primordial revealing the truth of the Precusors, his exile, loss of children in the Human-Forerunner war, etc eventually drive him mad (not entirely crazy, but messed up, anyway) leading him to do the things he does in the Halo 4 terminals, namely Composing a large amount of humans that the Librarian was caring for to raise his own army to defeat the Flood.
The Halo Array was the Master Builder, Faber's solution to the Flood. Shield Worlds/Composing Humans were the Ur-Didact's.
Afterwards, Mendicant Bias and The Flood have driven the Forerunner to their final, last stand at The Greater Ark, smashing the fuck out of it and the rest of the Forerunner fleets and Omega Halo. The IsoDidact is almost killed, if not for being rescued by 343 Guilty Spark, who then brings him to the Lesser Ark to activate the second, secret, more efficient Halo Array (the ones we know today).
The Librarian sneakily follows the Ur-Didact back to Requiem, then "betrays" him, attempting to force him to meditate within his Cryptum using the Domain and all the knowledge it possesses, until the humans can once again rise. She believes that by learning from the Domain, and from Forerunner knowledge, she can bring him back to reason from his self-torture and that he will eventually help the Humans.
The Librarian then leaves for Earth and tells the Flood fleets essentially to "come get her" to give the IsoDidact more time by distracting the Flood. Some Flood go to Earth and tell the Librarian that it knows the location of the Lesser Ark already (unlikely, and probably a bluff, since the Flood finds Installation 00, the Lesser Ark, in Halo 3). It also tells her a long lost truth: that the Domain, the source of all knowledge and interconnected amongst all Forerunner (basically their Internet/History), is actually a Precursor artifact, The "Organon" mentioned in Cryptum, and that it will be destroyed once the Halo Array is activated, destroying all Forerunner, and thus a very large portion of their knowledge, along with the Domain. The Librarian attempts to contact the IsoDidact to make him aware of this fact.
Back at the Lesser Ark, the IsoDidact disregards the message as a fake, and asks 343 Guilty Spark (and thus Chakas, one of his lost human friends) if it were his choice, would he activate the rings.
The IsoDidact lights the rings, destroying all life in the galaxy except the Ur-Didact in his Cryptum in Requiem, but since it also destroyed all Precursor artifacts, and the Domain, the Ur-Didact ended up being condemned to his own thoughts and madness for 100 000 years, rather than meditate within the Domain like the Librarian intended.
He pissed.
Great summary, thanks.