I believe what happened to Optimus Prime already happened a few times before in previous Transformers Films with all the other vilians we have seen, and here is why. I am going to skip the first two movies, as they have little to nothing to do with the theory.Now, in Dark of the Moon, Sentinel Prime comes back from the dead by the power of the Matrix. However, something seems off with him. At first, he is his normal self. He tells Optimus that he misses Cybertron, and that Earth gives him memories of it’s Golden Age. Sentinel in this scene seems geniune. He is not faking his emotions. But he knew that he was going to have to betray Optimus, which is why he knew that he was not worthy of the Matrix of Leadership. That is why he hands it back to Optimus, knowing that he may be their only hope. He grieves very, very much, as we see the look in his eyes as he is about to take down Ironhide, who he knew would pose the biggest threat to him besides Optimus. He did not attack the other Autobots (except in the comics he also killed Skids and Mudflap, which I have no problem with), because he knew this was already hard to bear. Then he is forced to work alongside Megatron, who gets the wrong idea and believes Sentinel is working FOR him, to which Sentinel reminds Megatron that he will work with him, but never work FOR him, and that he would be wise to remember the difference. Then, He intends to rebuild Cybertron. by bringing it time to Earth. This makes me think, before Sentinel Prime ran from Cybertron, he ran into the Creators, who told him the truth about where the Cybertronians had come from. They did to him what they probably did to Optimus Prime in The Last Knight. Basically they are the reason why Sentinel Prime betrays Optimus and is sorry for his actions. He even says the EXACT SAME THING that Optimus says in The Last Knight trailer: “Forgive me.” Forgive me. He did not want to do it. He did not want to betray Optimus. Just as Optimus did not want to betray Bumblebee and the other Autobots. He even attempts to explain his actions to Optimus at the end of the movie, even though he was murdered by Optimus before he could explain anything more. The Last Knight Trailer really gave us all a shock. Optimus seems to be in the EXACT SAME position as Sentinel Prime was in Dark of the Moon! He needs to revive Cybertron, and brings it to earth to do it! On a side note, you could also say that Lockdown went through the same thing. That is my theory thus far. Let me know if there are any holes in this, as this is only a theory. Discuss this on boards!