It's been some time, but here's another theory on the Forerunners and the Flood.
Rehalo (wow_sungun@hotmail.com) writes:
I have a theory: Evolution only keeps going when theres competition, when theres an enemy to bang heads with/kill/eat/become better than. If/when a race reaches a point where it no longer is threatened by anything other than itself, it will naturally implode and die. Yes. Die. But in order for a race to reach this point, it has got to be insanely well-developed and ultra-smart, something the Forerunner were. Cause lets face it, they made big rings in space that have the power to destroy big parts of the universe.
Brings me to the flood: The description of the multiplayer map "Warlock" says that this most likely was some sort of forerunner "Arena", that this was a place where battles and fights took place while other watched. (I'm partly guessing this)
Think about it: The Forerunner have discovered everything undiscoverable, done everything one can do with limitless power and knowledge, and basically turned every stone in the universe. Now, boredom is the worst enemy of this great civilization, that and the fact that there is nothing else to discover, reach for, or work with. So they invent a super organism specifically designed for fun and recreation, an "enemy" this super-civilization can kill and have fun with. But of course this goes wrong. The virus becomes too powerful, too resistant to any weapon. So now they have to escape their own creation, creating super-mega weapons that wipe out parts of the galaxy. They create an ark, blah-blah. The uber civilization of the forerunner is lost, killed by their own genious.
All that, OR the forerunner realized that there was no point in living and staying alive when there was nothing left to do/invent/whatever. So they make giant rings as a tribute to the holy dance of eternity, (Life and death, summer-easter-winter-spring thing) and kill themselves with it. They have reached a level of intellect and understanding that they no longer fear death, no, they actually embrace it, for it is all part of life, of nature. In the books, in the games you see it; the covenant ships use slipspace "engines" that instead of tears a hole in space, "unlocks" it. In the rock the spartans find in the fall of reach, cortana sees an "artistic" mathematical pattern. It's everywhere, the Forerunner knew nature so well they no longer had to use a hammer to beat the nail into the plank, they just had to think it.
Although this theory has been all chewed up, it never ceases to amaze me how new speculations arise. "A Flood to have fun with" ... I gotta get me some Flood then!