Why Did the Forerunners Create Halo?
A Group of Forerunners Conceived a Plan to Stop the Flood Once and for All, Building an Installation Known as the Ark That Created Seven Ring-Shaped...
A group of Forerunners conceived a plan to stop the Flood once and for all, building an installation known as the Ark that created seven ring-shaped megastructures called "Halos". The Halo Array, when activated, would destroy all sentient life within range—depriving the Flood of its food.
What killed the Forerunners?
Firing of the Halo ArrayEdit
They and all additional sentient life within three radii of the galactic center, died, as planned." — 343 Guilty Spark on the fate of the Forerunners.
Why did the Forerunners choose humans?
The Precursors decided that the second faction's (Forerunners') deviation from the original design was undesirable, so humanity was chosen to fulfill the purpose they were designed for.