Janet-- As for Barn not killing this or that person standing in his way-- I think it's a general tendency of people, even killers, to hesitate before taking such a big step. We might expect killers to do it much more readily after watching years of TV in which a lot of villains kill on impulse because they're "evil". I might be wrong-- how do I know, really?
Apart from that sort of human hesitation, murders would convince everyone in town there's far more danger than they realized, and increase everyone's drive to get the killer. Maybe the law thinks the kidnapper might have fled, and Barn would be wise to encourage them to think that... murders announce that he's still in town and determined to stay. (By the way, the faking of Maggie's death must be driving the town unnecessarily crazy.)
Not knowing specifics about the dialogue I can't say more, except in general, adding more crimes is sloppy and attracts attention. Also, I'm sure Barnabas considers himself a "gentleman", who would hold off from such acts unless absolutely necessary. Besides, all he has to do is make plans to off someone, and it gets all those dogs howling!
I'm babbling too much on this. I felt like posting again before trying to sleep, though.