Why kill him?
Harry was so gung-ho about trying to save Martin's life even when he was slowing the group down and Mitzi thought he'd end up a vegetable, so why did he decide so quickly (or at all) to mercy kill D.J. instead of trying to save him as well?
They could have made two trips to the cabin by the dam. That way the possibilities were that they'd either save D.J., or the killer would pick him off while they were gone, so why assure his death?