Good thread - thanks for the contributions.
Mindset is a complex thing, to be summed up in one simple word.
If I had to boil it down to one thing, I think it's a point Kathy already made (no surprise there.) That point being 'living in the moment.' ANY time a person is armed, they need to be fully in the moment, awake and aware.
Beyond that, it's the willingness to do significant violence when necessary. It's hard for a lot of people to shift their thought and behavior patterns from "nice person" mode into "killer" mode, and in fact is should be difficult. But the transition has to be mastered.
I've suggested Cooper's little book (Principles of Personal Defense) for decades, and have bought dozens of copies through the years (they keep disappearing somehow). I have yet to find anything better to introduce someone to the defensive mindset.
Mindset - Skillset - Toolset. In that order!
Attitude and skill will get you through times of no gear, better than gear will get you through times of no attitude and no skill.