The Reavers' true origin is revealed . . . in the movie Serenity. The crew of Serenity find evidence, in the form of an Alliance scientist's holographic message, that Reavers were originally humans from the planet Miranda. The Alliance government used Miranda as a testing ground for the chemical agent G-23 Paxilon Hydrochlorate, or simply "Pax" (Latin for "peace"). It was added to the planet's air processors in order to calm the population and weed out aggression. The agent worked, but too well: 99.9% of the population became so lethargic that they stopped working, talking and, eventually, eating and moving. They simply lay down and succumbed to death. The remaining 0.1% of the planet's 30 million people (approximately 30,000 total) had the opposite reaction to the Pax, becoming mindlessly violent and extremely aggressive. -- Wikipedia, Reavers.
If they take the ship, they'll rape us to death, eat our flesh, and sew our skins into their clothing. And, if we're very, very lucky, they'll do it in that order.— Zoe Washburne.
Karl Denninger takes on modern-day Reavers and the drugs that produce them: ENOUGH! (Guns, Active Shooters And Pharma)