In the movie Serenity, Mal and his crew travel to Miranda where they discover
that the Alliance had added a chemical called the Pax to the air supply on the planet. The Pax was designed to pacify the population and make them less aggressive, but the crew finds many bodies with no evidence of trauma or injury because the Pax caused most of the population to stop doing anything and simply die. However, they also discover a recording which reveals the origin of the Reavers -- the 0.1% of the population of Miranda which had the opposite reaction and became insanely aggressive.
Why didn't the Reavers murder and mutilate everyone on Miranda? Instead, everyone on Miranda
A plausible explanation is given in the Firefly Wiki article on the Reavers:
It has been observed/implied on at least two occasions that Reavers do not bother with already-dead prey, preferring, if not insisting, that their victims be alive. At one point, three Reavers were about to attack a man, but when Mal shot him, they gave up their effort. At another point, Mal, Zoë, Jayne, and River were on their Mule, being chased by a Reaver hovercraft. When Jayne questions why the Reavers did not destroy the Mule, River muttered to herself, "They want us alive when they eat us." Both these events take place in the film. In the same vein, the Reavers did not bother to attack the 99.9% of Miranda's population that were pacified by the chemical, as these people would have had no reaction.
However, the article does not appear to cite a canon source for its explanation. Moreover,
it would take days for the pacified population to die of starvation or thirst, and the Reavers would have had time to inflict massive trauma and fatal injuries to them. So why didn't the Reavers attack the pacified population before they died? Surely they would have taken sadistic pleasure in attacking the pacified population, even if there was no resistance. We only know that the Reavers ignore the dead, not the pacified.
Has a canonical source provided an explanation?