For his part, Sam clearly hopes that Jack can be redeemed in Dean’s eyes so they won’t have to kill him. Unfortunately, the odds aren’t really in his favor. Dean is a man with a plan motivated by what he believes to be the deaths of his mom and his best angel pal, and the losses of Mary and Castiel mean that Sam doesn’t really have any other allies who might be able to talk Dean down. There’s also the point that Jack might have been controllable with Kelly, Castiel, or Lucifer on hand to set him on a certain path. With Kelly and Cas dead and Lucifer trapped in another reality with Mary, it’s really just Sam, Dean, and Jack.