The first interesting research I came across is on how music influences life [Figone 2012]. It is a general article discussing how different music genres may affect not just humans but plants as well!
"What would my life be without music?"
Asks the author. [Feel free to express what music means to you in the comments section]
Classical music may be emotional with an ability to "strike the soul", whereas jazz "makes the groove". On the other hand though, heavy metal and rock is "well portrayed" since "the creator puts all their built up emotions in a bottle, and release the cap". The author argues that people may be captivated when listening to such music, mesmerized by the strong emotions and the connections they make with specific words or phrases.
This type of music develops into many different forms and may cause the release of a variety of emotions; from love and happiness to anger, fear, and depression. The author also notes a shift in the nature of rock songs from 40 years ago to today citing the transition from classics such as the "Bohemian Rhapsody" by Queen and "Don't Stop Believin'" by Journey to more explicit lyrics and aggressive musical compositions.
In general, major chords tend to make you feel good whereas minor chords tend to make you feel bad. Point is, music may have both good and adverse effects on human behaviour. It may promote learning, socially acceptable behaviour or even form a child's personality. On the other hand, music may promote bad feelings and antisocial behaviour going as far as conflict and war.
It is remarkable how heavy metal music affects plants. In a study, researchers found that plants increased their height by 30.7% when they "listened" to heavy metal and by 19.1% when they "listened" to classical music compared to "deaf" plants. The same plants also increased their leaf area by 27.3% (heavy metal) and 31% (classical) [Mynn & Shiqin, 2009]!
Author's spot on takeaway message:
"Music is magnificent, highly diverse form of art, and has the power to build or destroy".
My View
It is a fact that music may give rise to different emotions and promote a variety of behaviours. It is also a fact that heavy metal is very explicit with war, drugs, sex, religion, and fantasy elements. When a person listening to such music is equipped with decent thinking skills and is rational enough (I avoid using the word smart as it is misleading) they can easily enjoy the music and distinguish between reality and fantasy as well as between right and wrong. Also, when music facilitates the release of negative emotions in safe and controlled environments such as a concert venue or your own home, it can be extremely cathartic. I could go on for a while but all I say is that I refuse to believe that RATM's "Killing in the Name" is promoting more bad feelings or is more explicit than the stupendous "Rude Boy" by Rihanna (sample lyrics cited below).
Killing in the Name (RATM)
"Some of those that work forces, are the same that burn crosses...
Those who died are justified, for wearing the badge, they're the chosen whites
You justify those that died by wearing the badge, they're the chosen whites...
Killing in the name of!
And now you do what they told ya, now you're under control...
Fuck you, I won't do what you tell me"
VS
Rude Boy (Rihanna)
Can you get it up?
Come here, rude boy boy
Is you big enough?
Take it, take it
Baby, baby
Tonight I'm a let you be the captain
Tonight I'm a let you do your thing, yeah
Tonight I'm a let you be a rider
Giddy up, giddy up, giddy up babe"