For purposes of this discussion, metal is like guns. Guns don't kill people and metal doesn't cause psychosis. I've also heard that nazism, satanism, being a dictator and having wealth cause psychosis. Nonsense. The crazy person comes first, then attaches themselves to a symbol or tool that seems to them able to magnify their own ability to project willpower or forcefulness onto the external.
We have easy access to a great volume of information today, so it is more commonplace than ever before to see reports about psychos all over the world. Sometimes we hear they have a hobby or interest that happens to contain some extreme elements. This is coincidental to the extreme behaviour of the psycho, but it sounds like a more sensational news report for its inclusion. This is why we never hear questions about shoes, television, cars and houses causing psychosis. There is nothing considered extreme or powerful or evil about such objects, yet every other psycho uses them.