I suggest you point out to Beth that she is playing her PC as major EVIL alignment (I'd also debate the Lawful part as well). Make sure she understands that. Then whack her upside the head with the Paladin.
I'd also suggest you give a freehand to the Good PCs in the party in dealing with her. Frankly IMHO blantant mass slaughter of villagers for kicks is the sort of thing that that fully entitles good PCs to turn on a comrade and execute them. At the very least it makes it awfully hard to justify continuing to hang with her.
Stephen E