What if a 250 pound person punches you in the arm? What if a 150 pound person swings at your head? Are you going to pull your gun if a 90 pound woman starts waving a used needle at you?
Punches in arm? Likely not.
Swings at my head? Maybe.
Waving a used needle at me? Probably.
I think each one of those examples is also progressively more complicated.
It all depends on the totality of the circumstances. Use that term. It's used in law enforcement a lot, and it it's more than applicable to "civilian" life.
As an officer, if I perceive an imminent threat of death or serious physical injury to myself or another it is reasonable for me to utilize deadly force. For the average person they are often taught "if you are in fear for your life then you shooting someone in self-defense is justifiable."
Let's look at the facts.
If someone is punching me in the arm, with that fact in and of itself, then it would not be reasonable to utilize deadly force. You could fend off the blows. You could retreat. Your arm could hurt. If you have bone cancer and brittle bones then yep fire, but that's not a given here.
If someone was taking a swing at my head...that's a little more ambiguous. What is the person swinging with? Have I sustained any other injuries prior to this? Am I completely coherent at the moment such that any further blows to the head would be less likely to dull my senses? Will I be able to fend off the blows? Will I be able to retreat to a position of safety? Let's add to the scenario. If I had already been hit and was feeling a bit disoriented, I've been knocked down, and and in my attempts to get up I am unable to fend off the attacker, you bet I'm going to pull my weapon and continue to fire until the threat stops.
If someone is waving a used needle at me then I'm going to probably unholster and begin a retreat to a position of cover. I have no idea what diseases that person has, and, as an officer, being forcibly subjected to the body fluids of another person is a second degree (felony) battery. Regardless of that, it is reasonable to believe that the person in question may in fact have some type of transmissible disease that could have permanent and/or eventually fatal effects on my person. You bet I'd pull and fire my weapon until the threat stopped. Ok, what if it's known that the person does not have any diseases? That's a bit more complicated, and you'd more readily know that as her attending physician. However, there is a chance that the needle in question could cause serious physical injury. What if the attacker stuck the needle in your eye? That could cause irrepairable damage. Typically we define "serious physical injury" as anything requiring surgical intervention which is rather broad, and I feel gives one a lot of latitude. However, I believe an alternative use of force, as an officer, should be first employed in this circumstance. Most officers will have the tools and training to fend off a 90 pound attacker given everyday circumstances. However, in this case it's unlikely any common person or officer would know if the person had diseases, and the threat of that is much more apparent than being stuck in the eye so I believe firing upon the threat until it's stopped would be reasonable and prudent. If the subject waved a used needle at you I'd arrest her for aggravated assault. We also have a statute here pertaining to knowingly infecting another with HIV, etc., and I'd stack that charge later if I found it to be true and you were in fact stuck with it.
However, if I was her attending and she "waved" a needle I'd back out quickly and call for security. If she lunged at me I'd try to employ some type of defensive action such as a couch cushion, clipboard, or whatever you had available. A concealed weapon isn't usually drawn as quickly as one that isn't, and another person has already addressed the 21 foot reactionary distance (more or less). However, if given the opportunity, I'd draw whatever I happened to have secreted that day and shoot her.