Metal music began as the work of the youth born after the superpower age began, during a highly developmental period for Western civilization in which it, having defeated fascism and nationalism and other old-world evolution-based systems of government, considered itself highly evolved in a humanistic state of liberal democracy which benefitted the individual more than any system previously on record. During this era, society served citizens in their quest for the most convenient lifestyle possible, and any questions or goals outside of this worldview were not considered: it was considered a "progressive" continuation of human development from a primitive evolutionary "red in tooth and claw" state to one in which social concepts of justice and morality defined the life of the individual. The individual has triumphed over the natural world, and faces none of the uncertainty of mortal existence brought about by physical competition and predation.
Politically (the global quest for egalitarian society) and socially (the empowerment of new groups and loss of consensus) humanity viewed itself as getting ahead and being superior to other forms of civilization, including the equally egalitarian but totalitarian Communist empires of the Soviet Union and China, but as the thermonuclear age dawned in the 1950s, this dichotomy came to define the "free West" as much as its enemies.
The first generation after WWII created early proto-metal in a time when all older knowledge and social order was being overturned in the wake of an impulse to redesign the world to avoid the "evils" of the previous generation. The people of this age, and coming ages, were new in that they could not recall a time of direct experience of nature as necessary; the grocery stores, modern medicine and industrial economies of their time took care of all of their needs, and no unbroken natural world could any longer be found except on specialty tours. Their civilization had become exclusively introspective and was losing contact with the (natural) world beyond its self-defined boundaries.
During this time, a "peace" movement which embraced pacifism and egalitarian individualism was gaining popularity at the forefront of the counterculture, a phenomenon which had existed since in the 1950s smart marketers (namely Allen Freed) had promoted rock music as an alternative to the staid, traditional, monogamous and sober lives of Protestant, Anglo-Saxon Americans. With WWII polarizing the world against first German and later Russian "enemies," and Viet Nam revealing the moral bankruptcy of benevolent superpowers motivated by their economies, society was becoming more dependent upon the ideological tradition building over the last 2,000 years: focus on the individual, or individualism, as politically expressed in egalitarianism and liberal democracy. This was expressed in both culture and counterculture.
In contrast, metal music emphasized morbidity and glorified ancient civilizations as well as heroic struggles, merging the gothic attitudes of art rock with the broad scope of progressive rock, but most of all, its sound emphasized heavy: a literal reality that cut through all of our words and symbols and grand theories, to remind us that we are mortal and not ultimately able to control our lifespan or the inherent abilities we have. This clashed drastically with both the pacifist hippie movement and the religious and industrial sentiments of the broader society surrounding it.
This was a confrontation with the "abyss" as first described by existentialist F.W Nietzsche: the awareness that life is finite and of functional, transactional maintenance; that we are both predator and prey, and that we have no control over our lives or death. To Nietzsche, and thinkers such as Arthur Schopenhauer before him, to realize this was an "undergoing," or embracing of nihilism: the belief that there is no value other than the inherent, physical interaction of the natural world. To a nihilist, there is no inherent morality or value, thus there is no reason to view social status and financial success as ultimate goals, only as methods to a path ranked by subjectively-derived importance. This view threatens the beliefs and punishments used to hold Western society together since roughly AD 1000.
Regardless of benevolent social objectives, Nietzsche argued, religion and society were cults that banished death through the "revenge" that morality offered in giving the individual a vector by which to be "better" than the world itself, and by being "equal" to all others, immune to comparison (a symbolic form of predation triggered by Charles Darwin`s arguments on "survival of the fittest). In essence, Nietzsche saw social behavior itself as an enemy of reality recognition in the individual and thus, like morality, an ingrained influence that would prompt rebellion and instability within a society that would know no other recourse than moral norming.
Heavy metal, as the music most visibly fascinated with death and suffering (and most likely to mention Nietzsche), addresses the sublimated issue of Nietzsche`s abyss in Western society, which has based its founding principles and individual social and mystical values upon the polarity of "good" and "evil," is an identification with the enemy.
In the Judeo-Christian view, death and suffering are an enemy which is banished with "good" behavior in the hopes of heavenly (and earthly) reward. In secular form, egalitarian capitalist liberal democracy "empowers" the individual and gives him or her the moral "freedom" to act without regard for the natural world, thus being immune to predation and any form of assessment outside of the social and fiscal. When one embraces the breadth of history (outside of the current civilization), the nihilistic lack of eternal presence of value, the predominance of death and predation, and the logic of feral impulse, one has directly challenged both modern capitalist liberal democracy and the extensive religious (Judeo-Christian) and secular (liberalism) heritage upon which it is built.
8,000 years before Christ there was a religion in Northern India which addressed these issues in a sense without dualism; it believed that life is known to humans through sensual (eyes, ears, taste, smell, touch) perception of a reality composed of ideas which was similar in structure to both nature and the process of thought itself. In this religion the Faustian spirit was clearly present, as while a heroic deed was more important than survival, personal mortality was clearly affirmed. Thus there was both meaning and death, and no absolute God or Heaven to reconcile the two. This required the individual to declare values worthy of filling a life, and worth dying for, and from this origin the ancient heroic civilizations were spawned. Metal`s belief system is closer to this than to any modern equivalent, thus it is sensible to posit a closure of the cycle and its renewal in the ideas gestured by heavy metal music.