To quote my signature:

Black Metal is an expression of man's* collective immune system vomiting the degenerate religions and ideologies that have caused him spiritual sickness (by Casenius Cellarius, known as the user Infratetraskelion, now Stygian Cellarius)

*man of course means North Western European / Germanic humankind. Said from a true ethnocentristic view it is not necessary to mention that explicitly though

Speaking only for Black Metal (since I'm not that familiar with other metal styles anyway as that I could tell much about them), I think this statement hits the nail.

BM did not originate from metal, at least not alone. The protagonists of the early days were punks, albeit maybe only in their mindsets, rebels protesting against the social order. Unlike real punks who go anarchist, the Black Metal crowd rather sought a different kind of order. In fact, a rather traditional kind of order. Black Metal indeed is in all its facets an expression of disgust for "modernity", and likewise of disgust for cultural relativism. This was not a thought through and carefully laid out ideology though, let's not confuse this. This was an indistinct feeling of alienation from the surrounding world and its developments which brought forth a catharsis seeking reaction that eventually became a new music genre that also became a sort of ideology.

It is a diffuse one, one that centers around individual emotions and yet somehow is understood, or rather felt and shared. And although this ideology was present from the start, it developed a self-consciousness only over time. As the Metalcrypt article points out, it is a vision of another world and order that though does not exist and can only be experienced through allowing the cathartic emotions to be felt. The perspective from which this is viewed is inherently negative, and therefore black metal transports all those negative feelings, hate, aggression, frustration, pain, melancholy, depression, suicidal thoughts, because this social alienation leaves nothing but negativity in the soul of an individual, and dwelling within these emotions, allowing the pain and terror to be felt, generates catharsis, a little ease for the tormented soul.

This negativity though has never been the be-all and end-all of this. It's not negativity as such that gets promoted, through that is speaks to the primitive instincts, it only wants to make the individual react negatively to what is negative in its environment. Behind this negativity though is the longing for another world, a better world, not a paradise but only a world with another order, a simpler order. Without all that social engineering and pseudo sophistication and over-civilization that alienates the individual from its own social group, even from its very existence and life.

The early protagonists did not know this, nor would they have cared about a sociological analysis of their motives. They had only their emotions and a diffuse image of a still to be developed music style. And even today only few have grasped the essence of the beast they unleashed. And indeed they did.

They unleashed a beast that crushed the facade of "normality", it is the pain in the ass of society that goes a totally wrong direction, simply in allowing to let the emotional reactions to this wrongness to be the only rule, and to cultivate these emotions, or single aspects of these emotions into perfection and generate a contrasting value system that rejects modernity and replaces it with archaic, simple structures. Indeed, Black Metal is totalitarian, it is not democratic, because this is not our way to function. Black Metal is not humanitarian. From a humanist standpoint, Black Metal is misanthropic, nihilistic and anti-social. And Black Metal enjoys to paint out these aspects in every thinkable way. Because in reality, Black Metal is just not universal, the brotherhood of all races does not exist for us, we are ethnocentrists; we despise the sheeplike biomass mankind, regardless of race, and insofar are misanthropes to the bones, and most of all, we despise the resulting human from social (marxist) engineering that holds dear all these values that generate only disgust in us. Vomiting all the sickness, we enjoy ourselves to mirror this back to its source, all the sheeps in society that blindly follow a sick, pervert, universalist ideology. And Black Metal is elitists, because such an approach that enjoys itself in celebrating disgust, pain and psychotic breakdowns is something that is certainly not the right thing for the very most people. The entry fee to this elitist club is your naked soul stripped of every protectional layer, and indeed your life as you knew it. Black Metal is a way of no return, once in the world will never be the same again. And from there it only gets worse.