Some theists do indeed believe in a literal hell. They believe the threat of something someone doesn't believe in is a good way to convert someone into a false religion (or, put in another way, a religion). It also gives them pleasure to imagine the non-believers suffering for eternity while they're "safe" in Heaven. They're sadists, basically. They'd probably kill their family in the most painful way imaginable if they believed it gave them a better chance at entering the fairy realm.