And again, maybe Precursor CREATED the Flood, to prove who's worthy of the Mantle, the Flood went wrong, also infecting the Precursor, but because of their Trans-sentinent abilities they managed to merge with them. The Flood was somehow controlled by humans, and this made them worthy of the mantle. The Forerunner enraged and de-evolved humanity, eradicated Precursor, and took the mantle for themselves.
This theory would also explain the Trans-sentinence of the Flood