My opinion of Heavy Metal is the same as all genres of music and entertainment in general: it's your own perception and mindset that will dictate the outcome.
If you have some deep seeded mental health issues, be it depression, anxiety, anger or bi-polar disorders; whatever you see and hear will ultimately only make you feel worse. You could hear a Country song about a jilted lover and find yourself further in your own misery or watch the evening news and exit from it feeling absolutely hopeless.
But by the same notion, if a person isn't suffering from any obvious mental health issues, I think they could find happiness, joy, exhilaration and peace in any form of music, especially Heavy Metal. Heavy Metal itself for the largest part, is not a depressing genre but rather a very socially conscious one, tackling themes of the world around is in a realistic view. There are some bands out there that DO preach just misery and depression through their material, never writing about something positive but that problem exist again in all genres.
How many Country artists made their careers singing about loneliness, alcoholism, rejection and even domestic violence? ( Miranda Lambert and Carrie Underwood, im looking at your directions too!) Rap was engulfed in the gangsta image with drugs, gun violence and objectification of women throughout the late 80s and 90s yet maintained daily rotation on MTV. Pop even played with highly disturbing concepts such as mass shootings, drug addiction, sexual assault from the attacker's perspective and it's on the radio.
I think Heavy Metal is an easy Boogie Man because of the imagery more than the subject material. No better example than Iron Maiden! You take a casual viewer who really knows nothing about the band and let them see an album cover and they will begin piecing together this often dark image of an evil band who sings only of death, destruction and chaos, like the most wicked things you can imagine. Like this for example
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Yet, listen to them and its completely not what you expect. Often the songs are about historical events, novels and films. The lyrics basically narrations of said events, like an audiobook. They don't sing of depression, bleakness or demonic themes ( before you chime about Number of The Beast, its written about the film Damien: Omen II and a person trying to warn the public of the conspiracy and evil he witnesses, much like the characters in the film). When they do sing of war or destruction its from historical or literary perspectives. But if you judged a book by its cover, you'd write them off as just another evil band which they certainly cant lay claim too.
Again, everything is subjective to the viewer