Wolfish – There are two reasons why we kill a ????: a) ???? ????? – saving the victim, and b) ??????? ???????. Your tone of surprise is because you are working with the first reason, in which case killing him is obviously useless. However, according to the second reason, if he is about to commit suicide and there is no way of stopping him, you should kill him (provided suicide is ?? ????, as I said above).
Can you please cite where this is halacha? I would be very interested in reading such a ruling.
The Wolf