This is an excellent question, and, IMHO, one that anyone who carries a weapon in public should think long and hard about.
In my own state use of lethal force in defense of myself is justified if I believe I am at risk of death or grievous bodily harm.
Use of lethal force in defense of others is a different story altogether. What I *believe* isn’t relevant; the actual facts are. If the guy I see assaulting a female turns out to be a plainclothes cop detaining a suspect, I am screwed. Likewise if the person I believe is being assaulted by a violent partner tells a different story when questioned.
If I am traveling I will almost certainly be unfamiliar with local laws, so I have no intention of using deadly force (including drawing and brandishing a firearm) unless I am genuinely in fear for the life of myself or my beloved.
If I see a guy run into an elementary school carrying an AK I might have to re-think my position.