The Didact

He's insane.

He was a Forerunner, but then he mutated his own body and was cast out by his own kind, a scarring backstory strikingly similar to another guy no-one cares about. He was supreme commander of the Forerunner forces, and he had it all: Ships, Planets, Technology, and even the apparently beautiful Librarian was merely putty in his hands. But then came the Flood and the Reapers. They destroyed the Forerunner civilization and as each planet fell, the Didact became more and more desperate. In a final act of madness he decided to completely kill off everything in the universe, except the Flood and Reapers. Good Job.

The Librarian, being slightly smarter than your average Forerunner, went against the Didact and kept life forms alive, storing them on the Ark. At first he approved but as his sanity failed, he began to despise her. In the end she ditched him for Johnny, and the Didact became mad. He mutated his body to make himself awesome but he was then outcasted by the Forerunners and left to prowl Requiem for all eternity. Even Jorgey bullied him into giving up his best Composer, and Didact got stuck with the crappy one.

In the end, Johnny put him out of his misery by blowing him up in the charred remains of all that was left to him. How nice is that, eh?