(This is an AU where Megatron doesn't turn into a villain)

Back on Cybertron, after Starscream killed Sentinel Prime and took Sentinel Prime's T-Cog, which was Megatronus Prime's T-Cog, he turned the past members of the high guard into Decepticons, while Optimus Prime, Megatron, Bumblebee, and Elita-One created the Autobots to fight against the Decepticons, the planet was engulfed in war, killing it, so Optimus Prime, Megatron, Elita-One, and Bumblebee crash landed onto earth.