An adherent of any form of Satanism, a broad category of religions and worldviews all involving a favorable interpretation of "Satan." The most public form of Satanism is LaVeyan Satanism, which does not believe in or worship Satan as an actual entity, but instead sees Satan as a symbol of independence, individuality, etc. Since the late 1990's, thanks to the Internet, Satanism had diversified quite a bit. There are now more and more theistic Satanists, who do revere Satan as a deity - in most cases also associating Satan with independence, individuality, etc. Most do not see Satan as "evil" except in an ironic sense.
Many Christians have wrongly assumed that Pagans and even atheists are Satanists, who in turn are wrongly assumed to be violent criminals.