I have a theory about the Guardians. Maybe they were designed during the Forerunner-Human war as an "ultimate" version of the War Sphinx. Then, after the war, they were repurposed to be a form of Flood control, destroying flood infested ships and making sure that the parasite doesn't spread across the planet to form a Key Mind. But, unfortunately, they failed, and the Halo Array was put into service. That's my theory. The Guardians are possibly Forerunner WMDs like the Halo rings.