Why is mentioning this word the only crime punishable by death in this society? How does this word contradict the ideals of this society? What could its rediscovery possibly lead to?

Equality knows of an Unspeakable Word, and he knows that to speak it is the only crime with a death penalty. At the age of ten, he saw a man burned alive in public for saying the word. The Transgressor's tongue was cut out, but he was young and tall and walked proudly to the stake, smiling as he died. Equality 7-2521 had thought that he was a Saint, and he saw the Transgressor's eyes looking directly at him, begging him to regain the Unspeakable Word. Now, years later, Equality 7-2521 wishes he knew the Word, even if it means death.