Satan exists, or existed, but is not what Christians and Satanists believe him to be. Satan was not a single person, it was a title, a job. Any one of the elohim (the gods) could temporarily be a "Satan" to someone, to be a Satan means to torment someone. Basically Yahweh (who was himself one of the elohim) told one of his men "go be a Satan to that guy". After he had completed the task, he stopped being a Satan. Theologicians turned Satan into a character to serve their religion, like they turned other elohim into demons (because Yahweh had to be the one true god).
So, to answer your question, Satan isn't real and the cultists aren't insane, just ignorant.