Why Silence Speech?

"You can break a man's skull. You can arrest him. You can throw him into a dungeon. But how do you control what's up here? How do you fight an idea?"
--Sextus (Ben-Hur)

Why would people seek to silence speech they don't like? Several explanations are plausible.

One is that would-be censors simply can't handle the psychic pain of disagreement. Rather than reason with dissenters or simply walking away, they instead attempt to muzzle the unpleasantries.

Censors might also fear that those listening to dissenting voices are unable to think for themselves. When listening to independent viewpoints, those uncritical thinkers might believe what they hear even though, in the minds of the censors, that speech is obviously 'disinformation.' Because the censors believe that they have been enlightened to understand what the truth actually is, it is their duty to silence the disinformation and flood the pipes with 'facts.'

Of course, it is also possible that the censors fully understand that others are capable of thinking for themselves. They fear that, instead of blindly accepting the party line, listeners to opposing viewpoints will conclude that truth lies elsewhere--and that the truth will set them free. To keep people in the dark, censors seek to keep the light of truth switched off.

Finally, censoring is an essential tool of the propagandist. The central objective of propaganda is to undermine sense of truth. As successful propaganda campaigns have demonstrated, censoring seeks to keep truth out while lies are delivered in well insulated echo chambers. 

Censoring helps to keep the signal of the propagandists' message high compared to the noise of the truth.