When there was a conflict between autobots and decepticons; This dude had a serious influence on both sides. At one point, Sentinel Prime was trusted by the autobots to help end the war only to screw them over and kill Ironhide. It then turns into a hostile takeover when he links up with Megatron and the decepticons because he's the only one to stand up to the former and tell him he's the boss now (unlike Starscream). Out of pure jealousy and vengeance; Megatron and the Autobots are both out to take his head off. Dark of the Moon really felt like his story more than anyone elses. The previous two films really felt like a black and white conflict between two factions without depth or concrete context to it's origin. If it wasn't for bays genius to use Sentinel Prime as the conduit to piece their story together, I'm sure the average moviegoers would get tired of the franchise at that point.