It would be hard to determine in an unstable situation such as hostage holding, whether the person is going to harm him or not. The thing that keeps coming up for me is the reason only killing will result in freedom. If it were me in this scenario as hostage, I would try to get away...if I couldn't, and were in a position where killing them meant my freedom, I might do it, but would have to snap psychologically. That is they would have to break me internally to that point...like I would think they were going to kill me anyway or torture me or keep me hostage forever.
There would be no winner in that case, and it would be as if the dad committed two murders. If the dad did not take that position (as the cause of both kllings), then he is a moral ingrate. The moral responsibility is on him...not the hostage. If he is capable of love, then he will produce the evidence against himself, and save his innocent son, and get the framed person somewhat off the hook.