Harrison is seven feet tall, extremely athletic, and extremely intelligent. As such, the government and the Handicapper-General have attempted to use the most debilitating handicaps they can think of. In such an oppressive society, it would make sense to encourage and root for Harrison (or anyone) in his rebellion. However, he doesn't use his gifted mind and body to liberate society. He doesn't free others and try to take over the government or revolutionize it. Rather, he goes to a television studio and proclaims himself emperor. "Do you hear? I am the Emperor! Everybody must do what I say at once!" Instead of using his position to liberate others, he becomes more like a (temporary) dictator. The dancing scene is romantic but it lacks substance. It lacks any real attempt at rescuing this oppressed society.
We can applaud Harrison's spirit to revolt. And he does free one ballerina and some musicians. But he only does so in order to add to the spectacle of his own ego. The manner of his rebellion shows him to be selfish, only interested in showcasing his power. He, therefore, becomes no better than the government which uses its power to oppress the masses. Once he realizes his great power, it corrupts him. This underscores one of the themes of the story which is that power corrupts and that absolute power corrupts absolutely.