args.bungie.org

Also, what if the Flood was the creation of the Forerunner. Perhaps they attempted to create a new lifeform and found they were unable to kill "their offspring" even when it had proven itself evil?

An experiment gone terribly wrong —like grey goo.

Perhaps it's possible that the Forerunner were totally overrun by the Flood because they waited till the last minute to activate the rings... so most all of those "brave and honored souls" had already passed by the time "{D-Com}" made the decision to flip the switch.