If it helps, think of Reavers as less "zombie" and more "Hannibal Lectre on steroids". The Pax obviously was an endorphin stimulator; the behavior described in the film is exactly like that from the classic lab rat experiments in endorphin response.
Reavers came from that 1% of the population whose endorphin response is triggered by ABNORMAL stimuli: the sociopath/psychopaths. Think Ted Bundy, Jack the Ripper, et al.