Never got in to the metal thing myself, but I did listen to noise a lot and experienced similar surprises.
For example, Merzbow, has been around since the 1980's and his music sounds literally like a train wreck-- no conventional beat, no melody, no vocal, just random-sounding reverb, static, tape loops and sometimes machine noise. Similar stuff from P16D4, Asmus Tietchens, Zoviet France and others.
Many folks not familiar with this music would immediately assume it was all about these artists working through some really dark internal conflict, but that's not it at all. Instead, it was a kind of visceral catharsis, searching for a sublime authentic experience, rejecting the artifice of mimic-ing acoustically produced sound through electronics in favor of "native" electronic sounds that aren't bound to any natural reference.
I think that metal music fans have a similar kind of experience, except that the visual aesthetics of metal happen to have distinct associations that make it easy for outsiders to get the wrong idea. If you call yourselves "cannibal corpse" or do things like put skeletons, bloody tombstones, pentagrams and devils on your artworks, yeah, it's going to be interpreted (from outsiders) in ways that may have nothing to do with the ultimate intent of the music.
Merzbow frequently utilizes fetish and BDSM imagery / concepts and from the article perspective (although about death metal, it is similarly seen as "extreme music") I find that part of the article's conclusions interesting, in that for some there may be an endorphin release postulated sounds similar to what might be happening in the BDSM concept known as "sub space" (or occurring in other ways for other people, for example, "runner's high" is from what I understand a similar endorphin release).
As far as the top level comment goes though... maybe I'm misinterpreting the comment because this is summary level material, but yeah, that's an odd assertion to me. Death metal is a niche genre in a style of music that has some significant semi-mainstream elements (metal as a whole). Death metal is similar to other sort-of-tangentially related concepts like say horror media (there is a nice semi-mainstream side and also a lot of fringe horror movie / media styles that have fewer but sometimes more dedicated fans). Many horror movies for instance are flat out iconic at this point (ranging from the original Universal monsters to some of the films of Hitchcock to certain films like, say, Halloween, Jaws, or the Shining). On rare occasions, they are both quite popular and they even win an Academy Award (eg The Silence of the Lambs).
In music, the main thing I will say is that there are metal bands with often violent lyrics and imagery that are more popular (Slayer comes to mind), and non-metal bands with violent imagery that also are popular. Certainly if that violent imagery is slightly modified to a bit more theatrical, the band might even become mainstream (I'm thinking Alice Cooper as an example). The "sonic assault" of death metal in some ways wasn't new -- hardcore punk was an influence for the speed (although hardcore punk is quite a bit less technical) and there's certainly a couple relatively well known bands there in that genre that dabbled in (usually more theatrical but still macabre) horror. (The Damned and The Misfits are two fairly big names in the genre, not "mainstream" but big enough that some people will know of them, and there's plenty of other lesser known "horror punks" around...).
So to me that statement ("why are people interested in xyz that seems to induce a negative emotion when in everyday life we tend to avoid situations that will induce a negative emotion") is weird in this context, because this is not a niche question whatsoever overall. The answer to the above question is probably not best answered by looking at the psychology of an obscure genre and more best answered to ask why (in the Silence of the Lambs example) a movie that pushes "negative emotions" of this sort is (at present) #23 on IMDB's top movies of all time.
For example, Merzbow, has been around since the 1980's and his music sounds literally like a train wreck-- no conventional beat, no melody, no vocal, just random-sounding reverb, static, tape loops and sometimes machine noise. Similar stuff from P16D4, Asmus Tietchens, Zoviet France and others.
Many folks not familiar with this music would immediately assume it was all about these artists working through some really dark internal conflict, but that's not it at all. Instead, it was a kind of visceral catharsis, searching for a sublime authentic experience, rejecting the artifice of mimic-ing acoustically produced sound through electronics in favor of "native" electronic sounds that aren't bound to any natural reference.
I think that metal music fans have a similar kind of experience, except that the visual aesthetics of metal happen to have distinct associations that make it easy for outsiders to get the wrong idea. If you call yourselves "cannibal corpse" or do things like put skeletons, bloody tombstones, pentagrams and devils on your artworks, yeah, it's going to be interpreted (from outsiders) in ways that may have nothing to do with the ultimate intent of the music.