Megatron jumped Starsrcream's ass in one scene, and Starscream hates Megatron with a passion.
http://en.wikipedia.org/wiki/Starscream
Starscream makes no secret of his ambition to overthrow Megatron as Decepticon leader. He is more intelligent than the average Decepticon, ruthless and cruel in his actions, but also a coward — he is unlikely to take direct action on his ambition to overthrow Megatron without his fellow Decepticons to support him. He considers himself superior to the other Decepticons, and looks down at Megatron for being antiquated in his military strategy. Starscream believes the Decepticons should rely more on guile and speed rather than brute destructive force to defeat the Autobots, although even when Starscream is given the chance to strike out on his own, he is rarely more successful than Megatron.