As an avid death metal fan, I think that overall this article isn't wrong, but there's much more to be learned from the subjective experience of violent music. Which is to say: just ask a fan, and they'll tell you.

Using "negative" emotions to evoke a positive response is way, way older than metal. It's a technique as old as humanity itself, and is present the oldest stories we have (e.g, Homer, Gilgamesh and Beowulf.)

Storytelling and emotions are not straightforward things and the emotional response we get is not linearly tied to the subject matter.

To suggest that a metalhead must be negative and violent because they listen to Cannibal Corpse is tantamount to saying that someone must be suicidal because they enjoy a good performance of Hamlet.