"One" by Metallica reminds us that the most frightening horrors are man-made.
According to a common paraphrasing of a quote by French filmmaker and critic François Truffaut, there are no truly anti-war war movies; because war, when dramatised and fake, is inherently exciting. Thus, all war movies glorify war, whether they mean to or not. Anti-war protest music has a similar problem, but instead of glorifying war, it abstractifies war. Anti-war protest music is against war in the same way that it's against economic inequality or tax cuts for the rich, treating it like a political wrong, demurring from the gory reality of war.
Heavy metal does not demure from anything, let alone anything gory. It takes a certain distaste for subtlety to properly discuss war. This is why a genre not known for its eloquence is the only one that can clearly elucidate what war means.
See also "War Pigs" by Black Sabbath.
"One" by Metallica is brutally real
In fact, I cannot think of a war song as unflinchingly honest about war as "One" by Metallica (John Brown by Bob Dylan comes close.) I'm sure metal fans would be able to name ten others (and five more in German.) But you don't have to be a metal fan to be intimately familiar with Metallica's catalogue. Metallica is the rare example of an act that's been able to lodge difficult niche music squarely in the mainstream through sheer force of will, and "One" is one of their biggest hits.
"One" is easily the most horrifying song with regular mainstream airplay.
The song is based on Johnny Got His Gun, a novel by Dalton Trumbo about a World War I soldier who is left without a face or limbs after being hit by an artillery shell. Unable to communicate, he's effectively a prisoner of his own body. Eventually, he manages to communicate in Morse code, by banging his head against the pillow, begging to be killed, and put in a glass coffin in a freak show, as a demonstration of the horrors of war.
The video features scenes from the 1971 film adaptation of Johnny Got His Gun, and I vividly remember being not okay after seeing it for the first time,