There are many different, divergent beliefs on satanism.
I went from Christianity, to satanism, to where I am now which is basically watered-down, generalized spirituality.
What I, and the people I associated with believed, is that "Satan" is a sort of gestalt reality, an all-encompassing force that represents freedom. And since there is no afterlife, the world was we know it is hell. We are all in hell right now. Therefore, freedom of choice, thought, and action represents Satan, which represents freedom, and personal gratification.
Many satanists are something very close to atheists, and believe more of a philosophy than in mysticism and rituals. Some are more on that mystical, ritualistic side, but that eventually leads to paganism and wicca imo, and does not truly represent satanists.