I will freely admit that I have no idea. I think it does different things for different people. For me, the only time I really feel a buildup of negative energy is if the lyrics themselves, not the sound, are negative. For example, I love the power/prog band Powerwolf, great songs with a certain beauty to them, but when they start singing about Satan, even though they aren't Satanists and I don't believe Satan exists, those songs of theirs I have to skip past because I definitely feel it there.
Songs like the Amon Amarth song I posted don't give me any feelings of negative energy, quite the opposite. It invokes the warrior spirit of the Vikings and actually encourages me to get things done positively. I like to listen to them when I go walking because it helps me ignore the aching in my out-of-shape bones and focus on getting to my destination.
Like I said, the first three were OK. They remind me of Celtic music. A bit dour, but ultimately emotionally hopeful. The last two - much different. That kind of metal is deliberately angry in order to garner fans, but they don't understand what kind of energy those emotions produce or they do and don't care. It's the sort of thing that kills plants even in the absence of a human presence. People screaming in Beelzebub voices from hell, don't help. There is nothing remotely positive about that sound on a melodic or lyrical basis. To frame it in a biblical sense, it sounds like music from the doomed denizens of Hell.
"The difference between what we do and what we are capable of doing would suffice to solve most of the world’s problem.”