I think that you are largely correct on much of the metal image of "negativity" being a gimmick, and I agree that the music should be the main reason that one listens to a band. However, it nevertheless slightly irritates me that some bands would employ such powerful motifs in an inauthentic manner. Rather than have their art embody themselves (their actual values, beliefs, etc.), they employ them to project a fictitious image. In a sense, this seems to demean their art form.