He is bad in our point of view . He is a leader . He is a Prime . It would appear that when a Prime "falls" , they fall hard , much like The Fallen . These aliens' motivation was to save there planet and people which is noble enough , but would have been off the back of humanity . Optimus became a true hero , he was faced with a choice and chose humans and was willing to make the sacrifice , much like his Prime forefathers . Sentinel was willing to sacrifice his beliefs to save his planet Cybertron and themselves , no matter the cost . Sentinel would of become a Cybertronian hero had they won .