Al, I'm not sure if you're religious or not but I know you to be a thinker who's studied history. Most of my adult life I've tended to think of Satanists and the like as relatively harmless, free-thinking anarchist/artist types and not truly evil people. Was I lulled into thinking this by the media? Is Prometheus really Satanic?! Do you think this stuff is real or just some spooky theater stuff? My dad always said that Alice Cooper in the early days was one of the best shows he'd ever seen. Now I'm not sure what to think about all this. 
Sure a lot of it is for show or for shock value. Rebellion, controversy, and shock to get ratings and sales. As society decays though "shock" needs to get more and more "edgy" and "shocking" as it looses its shock appeal. And as social mores change or are eliminated. (For example now a star/movie/publicity-seeker needs to go "trans" or something to get attention because simply cheating on a spouse, 'swinging,' or homosexuality is passe.) Like sports, porn, 'entertainment,' sex, substance abuse, etc or often needs to become more and more "extreme" to maintain a marketshare or as an individual/society becomes desensitized and wants more.
A lot of it is for purposes of being allowed to do bad things (pedophilia, rape, frauds,etc.) For example someone might get into divination not really believing in it themselves but able to bilk money from people who do. Or the obvious cults for various reasons financial, egotistical, sexual, power-trips, etc. And with child abuse etc using satanic imagery etc can make the victim more scared to report the abuse (victim believe a large "ring" and their family is threatened) and if they do report the police/media won't believe it because it sounds so outrageous (aka "Satanic Panic" theory.) In addition sadists "get off" on the victimization, fear, and pain itself so use of scary images, rituals, masks, etc in the attack serves their perversion.
Then there are likely people who believe it -as there has been throughout history- and use the rituals, even human sacrifices, in some way to serve whatever Demon/Power etc. At that point whether 'you' believe it or not, 'they' do, and so it does damage to real people and/or society. As I mentioned above with regard to cults the leaders might not necessarily believe it but could gain followers who do so. Whether or not though, the actions do damage to people.
Also, such things can be used by enemies, foreign and domestic, to help fracture society. Things like the traditional family, religion, common values, beliefs, language, etc help hold society together and bond people together. So anyone seeking to destroy a country or culture would like to introduce things that disrupt, eliminate, or change these things. Undermine the foundation and the house crumbles. I say this, here, not as a value judgment and would apply to any society- think of the situation: when Western goods and media got into a Communist society, when Christianity got into Rome, when new technology or ideas is introduced to a primitive society, and so on. This is, of course, not necessarily a bad thing in some cases.
Regarding the 'reality' I guess that comes down to personal belief and how one attribute evil, certainly some of the more horrible crimes makes you think 'something' else must be involved because so horrible. Then again some would argue that humanity is by nature evil- to some extent or another. And there is always mental problems, drug-addled folks, etc who might not even know what they are doing when they commit crazy crimes. On the other hand like CS Lewis wrote in the "Screwtape Letters" (paraphrased) "the best trick the Devil did was convincing people he doesn't exist."
