Why do some people think that Satanists follow the biblical fallen archangel?

That is the only Satan they know... our gods whom have been demonized, lied about, in order to gain followers. Most of the gods they've demonized (they meaning those at the beginning of Christianity - most of these people don't know any of the actual gods and goddess') aren't "dark" at all... but to them, it's absolute evil. I don't think modern people realize just how ignorant they appear to be when they try to put down pagans because of our beliefs, based on propaganda from an old group of people who took up "lying for Jesus".