If you can justify taking action, take action. I have always felt that if you decide to carry, that means that you've agreed to the possibility of taking a human life if it becomes necessary. The issue is whether you decide to shoot or not, and why. You have to reason and justify in your head in about a half of a second.
For me, circumstances would dictate what I would do. There is no doubt I'm going for my gun, that's a certain fact. The question is when? I will go for it the instant that I safely can. If I am not seen, I will draw and get as close to the assailant as possible without being seen. I will then announce myself and pull the trigger as many times as it takes to drop the dirtbag. We had a trainer come in for a tactics related course who got into a shooting at a gas station. Thug enters and pulls a gun on the clerk. Cop was down one of the isles getting a bottle of pop and sees it, so he draws and sneaks up on the thug. Cop gets within a foot of the kid on the kid's 7 o'clock, yells "POLICE!" as loud as he could and then immediately pops the thug in the grape with a single shot. Game over. Store clerk told investigators that he heard the off-duty cop yell "police!" and then shoot the suspect. The cop said that he yelled "police" only for the benefit of the clerk.
This is ideally how I would do it. If I could not sneak up, I'd likely just hold as best of an advantageous position as possible behind any possible cover and send rounds downrange the instant that I knew it was a robbery or assault and that there was a weapon in play. Minnesota RECOMMENDS that a warning is given prior to the use of any force. As a cop, I would be judged in my actions like I would be as an on-duty cop, as I am using my gun under the authority of my department and our policies. As such, I am acting as a representative of my agency. Because of that, we're encouraged to give a warning if it is deemed feasible. However, this is rarely scrutinized in deadly force situations. All I have to do is explain that I was alone and had no backup, so I did not have anyone to cover me. I saw an immediate threat of death to the clerk, and if I made my presence known, my life would be in danger because action always beats reaction. If I lost on the reaction curve, I could be killed, or the clerk could be killed before I could shoot and STOP (
) the suspect. That's why I basically executed the suspect. Besides that, there was no guarantee that my first round would incapacitate, so I needed to make sure I had the most accurate shot possible for as fast of incapacitation as possible. Additionally, I know through my training that criminals will kill witnesses to avoid prosecution, and I had reason to believe that the suspect would kill the clerk once he got the money/prescription drugs/lottery tickets/whatever.
If the criminal had regard for human life, they wouldn't be threatening a person with gun or knife in the first place.