Megatron was not always the viciously powerful and brutally direct leader of the Decepticons: he was once brothers-in-arms with Optimus, appointed to be Cybertron's Protector and commander of its Defense Force. But Megatron resented his comrade, knowing he was a Prime and therefore Sentinel's favored son. This anger led to the creation of the Decepticons. It's not hard to imagine that Megatron would have erupted Cybertron into war to rid himself of Optimus.