Samhain / Halloween?

where did they get the idea that celebrating our ancenstors and dressing up to keep evil spirits away has anything to do with "Satan"? I still can't figure out why they consider satanist to be pagan in the first place. They should have their own clification since to be a satanist would require you to believe in the whole story of the Christian god having cast Lucifer out and all that, meaning in some twisted way they are an offshoot of Christian rather than pagan beliefs, IMO.