Why would anyone ever agree to kill the jailor?

This isn't a true prisoner's dilemma--prisoner's dilemma is straight decision-making with zero skill consideration. Cooperating is absolutely NOT the right thing to do in virtually any circumstance. The better player has zero incentive to cooperate -- it doesn't increase their chances of getting out. It doesn't increase ANYONE's chances considering that at any time you can betray and get a free trip back. There's no reason not to do that as the default move.