I wouldn't have the right to decide to take any one innocent life whether to save one or a billion. I would leave such things in God's hands.
If the one were a mass murderer who was currently spraying a crowd with bullets, I would hope I could find the strength to take him out, if that is what it would take. Of course, I would hope to find an effective alternative. Even the "lost child" with the gun might be saved.
Death of these Homo sapiens bodies we wear is regrettable, but not the most important thing to us or to God. But it's kind of hard to awaken spiritually without them.