Answer:
This is a sketchy subject, but to kill is an abomination. One the the ten commandments is to not murder, but,
lets say that you spotted a terrorist, and you have a gun. This man is about to kill thousands of people. Would you shoot? Most people would say yes. How is this any different from a single murder done by a man?
It is actually simple. Defense, you are defending yourself or others, and saving many more souls by killing one. Now, if there is another way to disarm the threat, by all means do so. but if your only option was to kill or let others be killed. The acceptable thing to do, would be, to shoot.
(Warning: This is only 119 words, you may add to it to bump it up to 200)