Music, if it's good, elicits an emotional response from the listener. As a result, it's always been blamed by some "moral" force or another for corrupting people. Way back in the day, women were sometimes banned from Beethoven's concerts because critics believed the music was too passionate and therefore a catalyst for corrupting their virtue.
This guy, like lots of others, developed a fanaticism over a particular band because the music made a connection with him. Unfortunately, his mental instability morphed this fan-aticism in an evil direction, made him pick up a gun instead of a guitar.
The music doesn't have to be heavy metal to have violent fans. For example, this tragedy happened on the 24th anniversary of Lennon's murder. His music was all about peace, but some unstable guy took him down too.
There's a wider topic in here, and that is, what is the inherent danger of celebrity, and what causes the fan mentality to sometimes turn violent.
On a side note, Dimebag's music kicks some ***. I saw Pantera about 10 years ago in Colorado Springs and wow, I still talk about it. The crowd-surge threw us back a good 25 feet when the music started. RIP Dimebag.