Most people agree art should be about expression, not repression. Whereas many cannot cope with the line of thought that says heaven and hell are both inside of the individual, nobody can deny that human beings accumulate repressed negativity throughout their lives. The deepest, most negative human emotions are kept hidden from others in order to be accepted and loved: fear, pain, sadness, anger, anxiety, violence, and frustration. From their early childhood, humans have been told to control them for the very practical end of becoming a functional being and avoid turning into a threat to society. Like it or not, this internal “hell” is part of each human; nowadays, it can finally come out and be embraced in a relatively safe way.
For example, many agree that dancing is not only entertaining but also liberating. The same principle applies to moshing and headbanging, which act as ways of expressing “hell” with sporadic aggressive movements. Under a philosophy that consists mostly on accepting or even adoring the obscurest ideas, metal decided to rebel against the alleged brightness of the 20th century achievements by touching the most hidden dark thoughts and desires. The wish of killing or dying is actually pretty common, together with some other thoughts that are usually repressed in order to belong to a certain society.
From a point of view, metal culture was born as the acceptance of those emotions; instead of directly taking us to kill a person, it focused on filtering violence through a particular arrangement of lyrics, rhythm, voices, and distorted guitars. Attending a metal performance is, for many, a kind of catharsis. The guttural voices could be perhaps a way for inner monsters would come out.
Leaving aside the fact that conservative parts of society are usually judgmental about the metal culture, there were indeed many cases of Satanism and cult adoration related to this music genre. These episodes led to a negative generalisation which still persists. Nevertheless, the constant alteration of the first sub-genres eventually led to the creation of trends that deviated towards positivism, spirituality, or straightforward religion.