i just watched this episode last night in fact, or was it today? no last nite, late last nite which is why i had trouble remembering. anyways, i don't think he was trying to make her forget, if you remember he didn't know sarah was around than. if my memory serves me right, he did know there was a little girl around the area buut NOT his little sister. i admit i forget this part so i may be wrong. please correct me if i am indeed incorrect. anyways,
as for him not killing Sam, i don't think he had a reason TO kill him. yes he had earlyier threatned to kill him if she made any noise. buut that was than, also if you look at "David Ford" he's not a skinny little thing for him to pick up. maybe if Willie was with him he may have killed him soo than he can have help moving both the bodies.
another guess is simply this, he was debating on what to do with her, he was one minute threatening her.
& he hadn't yet decided on HOW to kill her. if you remember Barnabas had told her he'd kill her slowly. or something along the lines of that. or he just didn't want to bodies to have to have removed. cause for one thing "Maggie" was in a way considered dead by the police. buut "Sam" will be missed. killing Jason later on was a guy that nobody would miss. he was to leave town anyways. well i'm out of guessing for the time being. everyon here has good idea's on why he just didn't kill them both.