You use to be a decepticon but Megatron betrayed you and left you to die on earth. You were currently in your car form hidden away from the humans and outobots until you heard them, the autobots; Optimus Prime, Bumblebee, Ironhide, Ratchet and Mirage walking around and talking.

Bumblebee: “Optimus, what are we going to do if we find a decepticon?”

Optimus Prime: “kill it with no mercy because they want to destroy all of humanity”