From a recent ode to Slayer:
The resulting tune was, appropriately, titled “Aggressive Perfector,” and it ushered in an era during which the band ditched the fake blood and histrionic shock of their formative days in favor of head-down assault. Or, as Araya says, “We started out with devils and demons, but we evolved to focus instead on the true devils and demons of society.” Which explains why the band ditched the D&D-esque vibe of early records like Hell Awaits and Show No Mercy in favor of a scorched-earth vivisection of society’s bleakest moments, often pairing their musical blitzkrieg rush with a lyrical preoccupation with war’s atrocities. Songs like “Mandatory Suicide” (from 1988’s South of Heaven) and “War Ensemble” (from 1990’s Seasons in the Abyss) combined a musical gut punch with lyrical odes to the senselessness of conflict that, to many, signified that Slayer were a band of their times, commenting on the brutality of the pugilistic Reagan/Bush years. – The Phoenix
Protest rock is all crap and Slayer lost focus when they went to protest rock.
Complaining about events in society enslaves the complainer to looking for approval from others, which requires whining about feelings hurt, the tragedy of others, etc.
Describing life in mythological terms instead frames the combat as one that can apply in any situation, and requires no pandering.