I agree that no band has ever ruined metal, and never will.
But I think that the people who don't know about metal might someday ruin it. I mean, when someone who says they like metal, but like bands like BMTH, Suicide Silence, Despised Icon, etc. Because that's what people (at least where I live) think is death metal.
They even go as far as saying "deathcore is death metal". IT'S FUCKING ANNOYING. And when they ask what bands I like, which are
Cannibal Corpse, Hell-Born, Sarpanitum, Dissenter, Nile, Aborted, Zyklon, Asesino, Dying Fetus, etc, they just don't even know what those are... They only know about deathcore and metalcore.