By the power of truth, I, while living, have conquered the universe.
DeceptionCobra brought up an interesting point in one of his threads pondering MB's current state of being either good or bad. Now my immediate impression was "well he wants to atone so he must be good", but after giving it some thought I think there's more to it.
The Forerunner, such bastards! In Cryptum my suspicions of them being more to them than the grand overseers of the galaxy were confirmed, it was no revelation to me that they were utter wankers.
I can see the Forerunners becoming quite bitter that they've exiled themselves from the Orion Arm. They have left behind a shattered empire and left the Orion Arm for reasons apparently unknown, be it "following in Their footsteps" or some other reason.
But as we know, humanity was given the Mantle before the Forerunners left which made them Reclaimers. Now I'm not really in a position to know anything about Forerunner politics, but there would certainly not be a unanimous agreement within their government to pass down everything that was theirs to a race they once despised and looked down on. That would be like handing over control of the world to Soviet Russia.
So then, perhaps there was a larger plan formed? What if giving humanity the Mantle was only a temporary thing? What if they were nurturing humanity to rebuild their empire until they were strong enough to return and take it back and MB was trying to atone by helping the Forerunners do this. This could also explain why the Assembly didn't want humanity to use Forerunner technology.
Compare this scenario to Mass Effect. The Reapers leave behind their technology (Mass Relays, the Citadel etc) for organic races to use so they develop down the paths they desire (destroy the Collector Base, folks). I can see this being applicable to the Forerunners because they aren't going to be happy about going from the greatest force in the galaxy to pitiful shells.
Thoughts?