We seem to be talking about three different sorts of people (Keeping in mind that these are very broad generalizations):
Neo-Satanists:
People who are taking their leads from Anton LaVey's Satanic Bible, which was published only recently, early 1900s I think, which encouraged people to worship themselves as gods and used Satan only as a kind of metaphor for the desires and needs that drive we humans. They still perform black magic rituals and summon demons from the abyss, but tend not to put to much stuck in Christian dogma/doctrine. It should be pointed out that, when they sacrifice people, they tend to have the kindness to do it in effigy, voodoo dolls, symbolic objects, and so on.
Traditional Satanists:
These are the people that pop up in movies and sermons, the fire and brimstone, sell your soul, child-raping, masochistic fiends that had their beginning in the middle ages. They believe in, but don't necessarily worship, Satan in the traditional, Christian sense. I dare say that none of them are as bad as you have heard, though they are kind of weird. Most of the shock stories about Satanic cults smuggling away babies and performing blasphemous rites with them in the woods are just plain lies.
Posers:
These are the people who think it's kind of cool to hang around in the candlelight wearing all black and muttering the enochian keys. I lump must of horror stories about 'Satanism' into this category. Admittedly, they do kind of suck (No sense of humor).
All of these three types are kind of abrasive to Christianity, especially the traditionals. But, in fairness to them, the one thing that the three have in common, even the posers to a lesser degree, is that they are trying to break down the limitations humankind had imposed on itself over the years. I'll stop talking now, because I'm starting to ramp up for a conversion speech, but all of you people who've insulted Satanists before would do well to remember, that a lot of them are alright people.
By and by, Aleister Crowley was not a Satanist but, from what I heard, he loved to joke around like he was, and said a lot of great sayings that Neo-Satanist have latched on to, one of which being the do what thou wilt quote. Which he was quoting from some old school eastern philosophy which I really don't know anything about. But I believe the real, full quote is, "That thou harm none, do what thou wilt." Wouldn't swear by that one though.