Just as people of all sorts of different points of view say they are Christians, there are various sorts of people claim to be Satanists and they make the claim with varying degrees of seriousness, and they mean different things by it.
Aliester Crowley claimed to have been in touch with a Gnostic tradition of “Jersualem Christianity” which he claimed was the true original Christian tradition which was later “perverted” into “Romish” Christianity. There is some discussion of this in Sex and Rockets, a biography from about 1998 of occultist Jack Parsons written under the pseudonym John Carter.
Off and on cynics and freethinkers have invoked the name of The Devil without really meaning much of anything by it. Examples are Bernard Shaw’s character The Devil’s Disciple, who talks about Satan to upset his hypocritical Puritan neighbors, and Ambrose Bierce, who retitled his collection of cynical quips, originally called The Cynic’s Word Book, as The Devil’s Dictionary. A lot of teenagers into heavy metal find they can upset their parents in the same way.
Anton La Vey, who cribbed a good deal of his work from the athiest philosopher Ayn Rand, appears to have used Satan’s name for much the same purpose. Oddly, a good deal of his blather about self interest, ffree will and libertarianism could pass for a sermon on The Trinity Network if one just made a few cosmetic changes, such as substituting “Jesus” for “Satan” now and again.
Michael Aquino of The Church of Set appears to be in deadly earnest about being in contact with Satan, though he describes him as being a much different fellow than the Christian, Jewish and Muslim faiths make him out to be. It is said that he had his falling out with Anton La Vey when he announced that he had conjured up Satan and talked to him personally, and La Vey told him he hadn’t, because all of that stuff is just make believe.
At least since the 1970s some conservative Protestants have insisted that there is a widespread Satanic conspiracy in the United States, and that it engages in human sacrifice. While living in al college town in Texas in the late 70s I used to see flyers advertising speakers at local funadmentalist churches who would speak on how they used to be devil worshipers who drank blood and witnesses the ritual murders of babies–but they were feeling much better now, and you could take what they said at face value.
A number of writers became celebrities of sorts among evangelical Christians by writing books about how they had been Satanic high priests, or the love slaves of devil worshipers. One such woman was able to maintain excellent attendance records in high school at the time she says she was being forced to bear babies for human sacrifice. As such claims became more repeated and more strident, the main stream media, always glad for a source of lurid panic, picked up on some of these stories uncritically.
There also developed a movement among psychiatrists interested in recovered memories. A number of them found they could get testimony of involvement in such movements from hypnotized patients. In one notorious case, a woman being treated for an eating disorder remembered that her father, a Presbyterian minister in western Missouri, was a Satanic high priest who had gotten her pregnant twice, murdering both babies. It later developed that the woman was still a virgin, and her father had undergone a vascectomy when she was two. At last word he was suing the congregation which fired him.
The FBI published a preport some years ago in which it concluded that there are no well-established Satanic conspiracies, although lone nuts and loosely associated groups of nuts do take to spouting about Satan from time to time.
Summing up: l various people say they are Satanists, and they mean various things by it. Some of these people are dangerous. By contrast, nearly all witch hunters are scary and dangerous.