Reaver

To the people of the Core Worlds, Reavers are a campfire tale and bedtime story; to the people of the Border Worlds and Colonies, Reavers are very real.
Reavers are believed by most of the 'verse to be men that went insane at the edge of space and became savage. They stared into the void beyond and became what they saw: nothing. They gave into their primal nature and all that was civilized was discarded.

However, key members of the Alliance's Parliament know better, and the crew of Serenity eventually discovers their true origin. Most folks believe that Reavers are no longer human. It's easier to believe they have nothing in common with the rest of humanity.