Popular music was recorded and marketed as a Counterculture which opposed the normal, functional, and unexciting Culture that was dominant in society; by being outside of that which was in power, Counterculturalists argued, they were able to see what was "real" and to implement a "progressive" worldview in which moral correctness brought us gradually closer to a utopian state.
This marketing mirrored the process of adolescents, the main audience for popular music, who first reject the world of their parents, then once independent re-assess their own values, and finally, rejoin society on the terms of these recreated values. This determines "reality" as they will act to create it, based upon their values system.
While dominant Culture sought what was pragmatic, and Counterculture pursued the moral, metal music became its own movement because it could not agree with either of those approaches, preferring instead to try to seek what was "real," or meaningful and "heavy" (in the LSD-influenced vernacular of the time). Their approach did not aim at correctness, but assertion of subjective meaning.
Early metal bands, in emulation of popular music as a whole, hoped to discover what was real by finding out first what was not. This attitude, over the course of four generations of music, took metal beyond the grounds of "good" versus "evil" into nihilism, where nothing had inherent value or classification, but could be described in terms of experience.
Nihilism is a frightening belief system for those in societies organized by dualistic (heaven versus earth) and liberal (individualistic, egalitarian) societies, as it denies that our values systems are more real than events in natural reality. To a nihilist, truth is a way we describe some things in reality, but there is no eternal life nor eternal truth which exists separate from immortality. Nihilism means accepting mortality, and experience as what we have in place of a religious or moral truth.
These ideas exceed limits of social acceptability, which in a capitalist liberal democracy threatens the self-marketing which individuals use to gain business partners, social groups and mates. As a result, metal was forced to wholly transcend the artificial consensual reality shared by Culture and Counterculture, and to create its own value system including its nihilism.
Seeking the real, and not the moral, this value system in turn surpassed its own nihilism by moving from a negative logical viewpoint to an assertive one, looking not for something objectively determined to be "eternal" but for that which will be true in any age past or present, discovering through personal experience and acceptance of nihilism (a symbolic analogue for mortality) that which society will not recognize, completing the process of adolescence in a state of actual outsidership.