With reference to Billy Ong's letter and Dr Azly Rahman's commentary, I don't think that asking whether Black Metal is a threat misses a pertinent point about the music and associated sub-culture.

Heavy metal music of the 1970s was hard, loud and harsh but it was more in the vein of protest and rebellion against authority or plain indulgence and sex.

Compared to others, Deep Purple was all about fun, protest and cheeky jibes at certain figures like Mary Long but otherwise they were relatively harmless.

Black Metal, on the other hand, has this morbid fascination with death, satanic symbology, evil, nihilism, etc, which I believe reflects personality problems, inferiority complex and lack of self-esteem in its fans irrespective of one's religious beliefs, so it can't be something healthy or good, or even neutral.