For me (and surely others), the negative emotions, dark imagery, sometimes downright disgusting lyrical content and other such trappings are fundamentally intertwined with many of my feelings about the worse aspects of life. It's totally part of the draw. There are many, many facts about human existence I am deeply sad or angry & horrified about, and art which reflects a shared understanding of these kinds of things offers camaraderie in misery-- sometimes it leads to catharsis, but not always.
You can separate the music itself from these things, and there are artists who have done this, yet even then the pure music is has many loud, fast and/or violent-sounding sections, and if you're angry, it can act as a channel for that anger to feel expressed.
counter-counter argument: but of course, that's Norwegian Black Metal, very much NOT Death Metal. And you'd be right, and not even all NBM acts welded themselves to the subject matter. But there've been some metal bands who were really passionate about embracing (not 'disconnecting' from) the subject matter, and always a few who sought to become the subject matter, and one or two did actually do that.
I'm not sure where this professor got this idea, but I'd say metal is identical to any genre in the feelings it can give you, except that it is considered "extreme" or "violent" or "negative" by some outside observers. Just watch any interview with someone like George "Corpsegrinder" Fisher and you'll see that even the most grotesque death metal practitioners are disconnected from the subject matter outside of the music.