In' it is revealed that The Fallen, a Prime who turned evil, is the First leader and founder of the Decepticons, which would seem to imply the Decepticons were originally Autobots who turned evil.
The Fallen is the First leader and founder of the Decepticons. This was what it is revealed. The Fallen is a Prime. This Prime turned evil. The Decepticons would seem to imply the Decepticons were originally Autobots. This Autobots turned evil.