So what bar would you set?Please explain how someone can form a "reasonable belief that their life was in danger" in a fraction of a second and how you are comfortable with that being the basis of taking a life. Perhaps, "I would have done the same thing in that situation" is your conclusion. What a terrible, lowly bar to set.
You are a farmer and you walk out into a field and a large, powerful man has beaten your pre-teen daughter senseless, is armed with a knife, and is forcibly raping her. You come into the clearing a few feet behind them and he starts to turn his head toward you. There is a four foot section of 2x4 at your feet. What do you do? Call "time out!" and sit there and consider your options? Run away and leave him there with your daughter while you go back to the farmhouse to call the police, who will take at least a half hour to get to your place?
Perhaps the standard is what a reasonable person would do given a fraction of a second to make a decision is because, in many situations, that is all the time you HAVE to make a decision.