Suppose Jim physically restrains Janis, puts her between himself and Kurt, and then starts shooting at Kurt. The only way Kurt can shoot back at Jim is if he shoots Janis first.
When Kurt kills Janis, is it Kurt or Jim the person we should blame for Janis's death? One might argue that it is Jim who is morally responsible because if he had not used Janis as a human shield, Kurt would not have to defend himself and kill Janis.
But, while it is Jim who decided to use Janis as a human shield, he did not make the decision whether Janis will live or die. It is Kurt that made that decision. Kurt chose to save his own life by ending Janis's. He chose that outcome instead of the alternative: letting Jim kill him. If Kurt let Jim kill him, then Jim would be morally responsible for Kurt's death AND for restraining Janis against her will, but obviously not for her death, since she would still be alive. While Kurt is perfectly free to choose his own death, it is hard to escape the conclusion that he does not have the right to choose whether someone else will live or die.
Obviously, Janis did nothing to deserve being killed, so we can't derive Kurt's right to kill her from anything Janis did. Also, we can't derive Jim's right to Kill Janis from something someone else, including Jim, did. If we were to derive Kurt's right to kill Janis from a third person's actions, we would put Janis at the mercy of a third person. Would you want that someone's right to kill you be conditional on something I do when you have absolutely no way of controlling my actions?
If we claim that Kurt is not morally responsible for Janis's death, we are also claiming that Kurt had a moral obligation to save his own life. In other words, we are claiming that, had he not defended his own life, he would have done something morally wrong. Or, by defending himself he was trying avoid doing something wrong, which is choosing to be killed. Is it morally wrong to choose to be killed? I don't think it is. In fact, most of us think it is heroic to sacrifice one's own life for the sake of saving someone else's. On the other hand, most of us think it is selfish to take someone's else's life in order to save one's own life.
Instead of concluding, it would be useful to stress that both Jim and Kurt have choices to make, but Janis is completely helpless. Jim has the power to decide whether Kurt will live or die, and Kurt has the power to decide whether Janis and Jim will live or die. Therefore, the correct questions are:
1. Does Jim have the right to restrain Janis?
My answer - No, and therefore Jim is guilty of restraining Janis.
2. Does Jim have the right to choose whether Kurt will live or die?
My answer - No, and therefore Jim is guilty of Kurt's death if Kurt refuses to shoot, and Jim kills him.
3. Does Kurt have the right to choose whether Janis will live or die? My answer - No, and therefore Kurt is guilty of Janis's death if he chooses to shoot.
4. Does Kurt have the right to choose whether Jim will live or die?
My answer - Yes, and therefore Kurt is not guilty of Jim's death if he (Kurt) chooses to defend himself.