Satan is the adversarial force in nature that governs all life; it is observable, but unknowable, transcending both science and spirituality. Those who acknowledge, and embrace this force sometimes call themselves Satanic.
"Satan" itself is just a label, it's an ancient word that translates into Adversary. Some would argue it's ties to the Abrahamic archetype, but again, just labels. Some people like personification within their paradigm, which might also explain their reverence for such a character.
If we are to understand that self-preservation is Nature's highest law, then it stands to reason that Satan (the Adversarial force) is it's governor.