Unfortunately, I did not catch this documentary, as I only get BBC America and I did not know it was on. Else I'd have caught it. However I do have some opinions concerning some of the topics discussed herein.
Bands are obviously influential. Music is a terrific way to influence both cultural and individual behavior, and vice versa. In the 1960's, the American music scene influenced the culture and the culture influenced the scene. Sex drugs, decadence, hippies, etc. It is what it is because the people living in the culture make the music about the culture. The most frank example of this is early gangsta rap. People like NWA, Ice-T, Snoop Dogg and Tupac were writing music about the world they lived in- complete with rampant crime, drug use and police brutality. It became popular because the message was one of fighting the man, lashing out at authority, and indulging one's own desires. White kids who had never even talked to a cop, I'm sure, identified with this because of the simple message of rebellion. A scene was formed.
Now let's look at metal. It's the same message- fighting the man, lashing out at authority, indulging one's own desires. The best metal has always been about serious ass-kicking, no way around that. The main difference between the messages in rap and metal, however, is the spiritual aspect. I have heard plenty of metal talking about Satan and demons, but I've heard very little rap music on the subject. Tech 9ine is about the only the only one I know of, and his music is mostly about drugs and killing and fucking ho's.
But what is the effect of Satanic metal? It is somewhat of a scene to itself. If it weren't, bands like Deicide and Akercocke and Goatwhore and Dissection wouldn't be that popular. Does it make people sacrifice virgins to Satan or burn churches or draw pentagrams in goat's blood? I would say, no more than rap music makes people shoot a cop or beat up a woman. Every person needs to be responsible for their own behavior.
However, there is such a thing, in both morality and law, as being an accessory. If you coerce or ask a person to commit a crime, you are equally guilty. In the Bible, Christ warns against causing your brother to stumble (ie: commit a sin). It is not fair or moral to totally absolve these bands from the behavior of those who listen to their music. Like Uncle Ben said, great power brings great responsibility. If you have this wonderful freedom to spread your message of evil and Satanism to the masses, then you should not be surprised, nor absolved, when those who listen to it and accept your message turn out to make some rather poor choices.
The bottom line is, a good bit of popular music involves destructive and irresponsible themes. That's rock and roll, pure and simple. But there's a line somewhere that can be discerned, between what is destructive and what is evil. Whether you believe in evil or not, you must admit that the behavior reported of some of these artists (church burnings, murder, cultery) is dangerous and in some cases unlawful. It is the same for rap music- drugs, guns, violence, objectification of women- the difference seems to be that black people never blame Snoop Dogg for their weed habit, or Old Dirty Bastard for when they bring a gun to a club.
There has, as of yet, been no real way (at least in America) for any authority to deal with those 'accessory'-type influences. Censorship, beginning with Elvis and mostly ending with Marilyn Manson, was an utter failure. Throughout the years, censorship has only made its 'victims' more popular, and in many cases icons. In a free society, any artist can be as destructive or lecherous as they please, and the leavings of this influence- the resulting behavior of the listener- is left entirely in the listener's hands.
Ponder this for a moment: A band records an album, but secretly records a track under the sound that is inaudible to the ear, but reaches the subconscious and influences the listener to commit a crime or harmful act. After this act is done, who is responsible? If it is the band, why are they responsible when the influence is plain to hear, but not responsible when it is not?