gnostic
The Lost One
A lot of atheists who I know, don't believe in Satan any more than they believe in God.Within Judaism Satan is an angel on G-d's side, so it also makes no sense from this context, but most Satanists don't identify with the Christian Satan. A lot are, in fact, atheists.
To them, Satan is just as mythological as Yahweh, Allah, Zeus, Gaia, Odin, Marduk, Brahma, Shiva, fairies, elves, vampires, etc.