Yes, self defense, acts of war and stopping someone from the commission of a crime (say throwing a lit molotov cocktail into a crowd) is a reasonably good answer to "why".<quoted text>
You can get away free with killing another person, or you can go to death row, depending on the answer to,..."why?".
Powerful, for something that doesn't exist.
Anger, greed, jealousy, criminal negligence, etc., are not really answers, they are merely rationalizations. That's why we have laws saying which "why" lets a person go free or to death row.