Megadeath is a secular group, their music has a secular connotation, and their songs are not liturgical or meant for that type of forum. Regardless of one’s intentions, Megadeath is inappropriate for the Mass by its very nature.
To the extent that the music in question has a secular origin, that
could disqualify it as inappropriate for sacred use. However, stating that music from a secular source is automatically disqualified is a weak argument. I’m not a musical expert, but I believe there are some hymns that originally had a secular purpose/content. Since we don’t associate these religious hymns with their original connotations, there’s no problem.
There could also be the opposite occurrence; a sacred piece could be adapted as the theme for a very popular movie, television show, or commercial completely inappropriate for any association with the mass. It’s predominant connotation in a culture could thus render it inappropriate for use in mass.
An example of the former was Wagner’s Wedding March. Unless one is enough of an opera fan to know it’s origin, it’s likely to simply connote matrimony. As its origin has become more widely known, there is more cause for scandal and more likelihood of inappropriate associations with the music for those attending the event, so it’s been prohibited in a number of places.
I believe a frequent replacement piece is Mouret’s Rondeau, which always conjures in my mind Masterpiece Theater rather than the Sacrament of Matrimony. Another sacred piece that is no longer appropriate for me personally is Just a Closer Walk with Thee. I wasn’t familiar with the song before seeing Peter Jackson’s tragic film Heavenly Creatures.
So a piece’s secular or sacred origin is only relevant if that origin is understood and retained by the culture in which the mass takes place. In the case of Megadeth in the US, the answer’s obvious.