User blog comment:Subject 9/Reasons/@comment-1625009-20110905085222

the Reapers could also be ancient warmachines, following the only purpose they ever had, Kill everything. but they adapted it in order to continue having a purpose -> kill everything in space.

Would be a good plot-twist: the reapers follow the same cycle for aeons, because they need a raison d'être and have no way to evolve beyond their current state.