It costed Chief Cortana to stop the Didact temporarily, his best and possibly only friend gone to delay him. I feel like in the end Chief will give his life to defeat him.
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I believe the Didact's speech was during the last Forerunner-Human war, however it could also be that he has joined up with other living Forerunners and is now attempting to convince them that Humanity is a serious threat to the mantle; that we will destroy it rather than uphold it. The Didact is more than likely alive I believe, and Cortana is with him, trapped inside his armour. The question for me is; will he emerge an enemy again, or an unwilling ally like the Arbiter in Halo 2?