When I was a kid, Metallica was widely seen as completely crazy (this is the Kill 'Em All to Master of Puppets era). "Big kids" who listened to Metallica were vaguely threatening and best avoided. Now even those early thrash monsters are considered to be classic rock. Metallica is eminently respectable.

There is some extreme metal that will never be mainstream or widely listened to but that mainly comes down to the amount of work that's required to listen to it (Deathspell Omega) or the purely misanthropic production quality (Darkthrone's Panzerfaust, an album I adore). This sort of music is "shocking" in the sense that most people assume bands make music because they actually want people to listen to it.