You will most likely have to convince a DA or jury that you, or the person you protected, were in immediate fear of death or serious bodily injury. You don't always have to think you are going to die. You can defend yourself from serious injury although defending yourself in court could be more difficult.
You will also likely be judged against some variation of a use of force continuum. Justification for the use of deadly force can be due to a disparity of force like multiple assailants, an assailant much larger or more fit or one with a weapon.
It will also depend on your State. Some require you to retreat until no other option is available and some do not. If you are in a situation and you are wondering if the circumstances warrant the use of deadly force then they probably don't. If the use of deadly force is needed you will most likely be fully aware of that fact without having to ask yourself.
For me personally, I will not change what I carry for fear of prosecution. I intend to do all I can to survive and I will deal with the court when that time comes. I think prosecution trumps burial.
IDPA Member A00640
Founding Charter Member - Middle Tennessee Shooter's Club