User blog comment:GodzillaMaster/Theory on the Reapers/@comment-2017893-20110829045455

Not to get out-of-universe but frankly I think the Reapers are most analogous to CLU (from the Tron universe.) Kinda like the Geth on steroids. At least 37 million years ago (remember, nobody said the derelict reaper was the oldest reaper, it's just an old, disabled one of many), a normal, organic race created a race of machines and gave them a single directive: create the perfect system...err, universe, I guess. This race - the Reapers - did exactly as they were told. Starting with whichever organic race created them, and on a 50,000 year cycle, they chose the "best" race of all the organic races in existence, fused it into a new Reaper, and then wiped out all organic life and waited. My guess is that only when they wait 50,000 years and there is no new sentient organic life is their mission complete. At that point, they should analyze and compare all the reapers then in existence, and decide which is the closest to perfect. Once decided, all the other Reapers should self-destruct, leaving only the one most perfect Reaper.

That's my theory anyway. Seems like they're just fully unrestrained Geth to me.