Remember that comment a few weeks back, about how a violent death should serve a purpose to the story? Now this entry on the list assumes that this is going to be the final installment in a trilogy despite the director's noncommittal comments on the future. If this is indeed going to be the end, then we need to bid our hero adieu, and just like so many heroic tales, the most logical conclusion for his long-time struggle is martyrdom. Meanwhile, the apes go on to fulfill a destiny only he could have made possible. Audiences are going to need some sort of finality in a story with an otherwise limitless future. Whodunnit and why is anyone's guess, but the possibilities are interesting. Perhaps the humans did it, which sparks a response from the apes, or hey, maybe they introduce an ape named Brutus. What you can't have is Caesar dying in his sleep, and the apes just carrying on, which would horribly diminish his impact (and the preceding four hours of cinema.)