There is absolutely nothing to justify killing - anyone who says that it is ok if they are defending their country or other people, are fooling themselves.
You have heard that it was said, 'Eye for eye, and tooth for tooth'. But I tell you, do not resist an evil person. “You have heard that it was said, ‘Love your neighbor and hate your enemy.’ But I tell you, love your enemies and pray for those who persecute you, that you may be children of your Father in heaven. He causes his sun to rise on the evil and the good, and sends rain on the righteous and the unrighteous..."
You cannot justify killing whatsoever ! Can you love your enemy and kill them??? My goodness, whoever says otherwise has distorted the message of Jesus Christ very much.
When they came to arrest Jesus, St Peter drew out his sword, and Jesus rebuked him! How can you people not understand the message that there is no such thing as a "justifiable" or "just" war.