In the same way that the vast nebulous world of "fandom" has its own anal-retentive hardcore fanboys, the folks calling themselves "Satanists" have their own cadre of hardcore "purists" who look down on other sects with a sneer, because they know they are the real Satanists. Michael Aquino's Temple of Set is known for bickering with other Satanists and atheists just as much as they bicker with Christians, if not more so.
Officially, Aquino broke away and founded the Temple of Set because of Anton LaVey's move to sell priesthoods in the Church of Satan for cash. This upset Aquino greatly, and he subsequently left to start his own organization. He claimed that the Church of Satan had become ridden with "corruption" (a cult of devil-worshippers being too corrupt?), and hence the Temple of Set is intended to follow the "true path."
Of course, a number of Satanists will also suggest that Aquino's problem was one of anal retentiveness. A number of of the group's members (who use the term "Xeper" to describe their philosophy – "Xeper" being based on an Egyption hieroglyph meaning "enlightened individualism") are known for having little sense of humor when it comes to their group, and they especially despise the attitude of some Satanists to laugh at themselves along with the rest of the world. "Satan fundamentalists" may be a way to describe the Temple of Set – although they would most likely disagree with this statement.
Aquino himself has been hounded by some very nasty accusations, and the Temple of Set has filed lawsuits against individuals who have fleshed out these accusations in print. The group was also known for trying to silence criticism on the Internet in the late 1990s, using "Scientology-like tactics."