Equality is supposed to be a positive thing, but Vonnegut treats it in a negative manner because in this world, equality is achieved through harming individuals.
If equality can only be achieved through handicapping, this is not equality at all. Harrison Bergeron's "perfection" is a threat because he proves that it is impossible to achieve true equality. Even with the handicaps placed on him, he was still born taller and better looking than most people. The handicaps cannot control reality.
In this world, the idea of equality is taken to the extreme. Vonnegut is exploring what happens when the notion of equality is taken too far. It is impossible for everyone to be truly equal; there will also be someone better looking, more talented, or smarter -- something proven by Harrison Bergeron's very existence.
