The society that Harrison comes from stifles any sort of individuality, freedom of expression, talents, strengths, gifts or abilities. Harrison has spent his entire life being told that he needs to fit in, and that his gifts and physique make everyone around him feel inferior. Being told that over and over, and being given such difficult handicaps to deal with, must have been very difficult and frustating for Harrison. Here is a kid who, at fourteen, was "a genius and an athlete...[and] exactly seven feet tall" who has also spent a good part of his life trying to be himself. Those attempts to be himself and to reach his potential have landed him "in jail," from which he escapes for his final escapade on the dance floor.
So, imagine being shackled your entire life and then to all of a sudden be free, dancing with a beautiful girl. You would probably feel completely elated and joyful, and like you were on top of the world. In comparison to the miserable, shackled life you led before, you would feel like an emporer. You would feel powerful, strong, and like you could do anything. So, here Harrison was, dancing to beautiful music with a beautiful girl, free from jail and all of his handicaps, for the first time in his life: he had to be feeling good. And that feeling was probably what prompted his declaration that "I am the emporer." He wanted to show everyone what he "could become," his true potential. And, he did, for a moment, before his demise.
I hope that those thoughts help a bit; good luck!