It all boils down to whether you can buy that an atmospheric gas made Reavers psychopathic towards normal humans, but not each other. I don't buy it, for reasons stated above, and I for one have said my peace on the matter.
Well, it wasn't so much an "atmospheric gas" - it was a psychotropic drug distributed through the planet's atmospheric systems. Best guess is that they didn't know all of its side-effects, obviously - what if it also adjusts the body's production of, and sensitivity to, pheromones? Perhaps as part of the pacification, intended or not, the body produced specific pheromones that would promote peacefulness amongst the group, but in the Reavers' case, it created a 'pack scent,' so they can recognize one another and cooperate, but are excited to aggression by the pheromones of the 'sheep,' or at least by the lack of a pack scent?