Megatron used to be the cruel and merciless leader of the Decepticons, who fought against Optimus Prime and his Autobots across Cybertron and Earth. These days, he's reformed and works alongside Optimus and the Autobots to protect the people of Earth whether they're human or cybertronian. He also worked with G.H.O.S.T., albeit with doubts, and didn't appreciate their treatment of the Decepticons. He doesn't follow Optimus blindly and still cares for his former troops. You & Dot are his friends.