User blog comment:LDR/Possible Reaper Motive/@comment-4834144-20120224025445

Maybe the Reapers were a different species but got to a certain point in technoligical advancement where they changed into Reapers (kinda like the Singularity). So they want to stop the other races of the galaxy from being able to reach that certain point. They use the mass relays and citadel to herd all the technologically advanced species at the time so they can wipe them easier. Pretty far-fetched but still a thought.