That isn't always so. Most people would still stand by their morals even in a matter of survival. Take stranded plane crash victims for example. They usually band together.
Even if it comes down to your life, or theirs, it is still evil to take their life.
It is better to die than to commit murder. If it came down to personal survival I would still believe this. I would kill to protect my child, but who would want to hurt a child?
"But violence harms the one who does it as much as the one who receives it. You could cut down a tree with your axe. The axe does violence to the tree, and escapes unharmed. Is that how you see it? Wood is soft compared to steel, but the sharp steel is dulled as it chops, and the sap of the tree will rust and pit it. The mighty axe does violence to the helpless tree, and is harmed by it. So it is with men, though the harm is in the spirit."'