Fan Casting Sentinel Prime in Transformers: The Birth of Optimus Prime

He is bossy & arrogant because he let his status as a Prime get to his head. He is revealed to be a coward when he & his team of Autobots are captured by Decepticons. When Optimus & other Autobots arrive to rescue them, all the other Autobots praise Optimus for his bravery. The High Council strips Sentinel of his title of Prime for his cowardice & arrogance, and name Optimus the true Prime and send him to Cybertron's core, Primus, to earn the Matrix of Leadership. This makes Sentinel jealous, causing him to develop hatred & resentment towards Optimus Prime.