Excused Necessity in Western Legal Philosophy

Should a person in a life-threatening situation have a defence when he saves his life by causing death to a person who was not involved in creating the life-threatening situation? Viz., does the perpetrator have an excused necessity defence that negates imposing the punishment? This issue - excused necessity defence - has fascinated the legal world since antiquity and has been described as one of the most complicated issues in criminal law. The well-known case is the “plank of Carneades” or “two men and the plank”. Two men, A and B, are shipwrecked on the high seas; as their strength ebbs and they are about to drown, they see a wooden plank that is just large enough to support only one of them. A reaches the plank first and grabs it, but B, faced with the prospect of certain death, pushes A off the plank, resulting in the death of A by drowning. B then grabs the plank and manages to save his own life. Should B have a defence if he is prosecuted for pushing A off the plank in these circumstances?