I’m not here to second guess the slain officer’s actions prior to his murder. I’m simply pointing out the fact that there are subconscious cues indicating the onset of violent behavior from people who are not overtly aggressive. An armed (and unarmed) citizen should remain alert for the possibility of an attack whenever they are in a “discussion” with someone they don’t know very well. If the person appears unresponsive or listless, not making eye contact, it’s always best to put some distance between yourself and the other person. skillsyouneed.com provides a list of warning signs, including and especially any history of violence . . .
Frustration (e.g., from an inability to communicate effectively).
He/she feels threatened.
He/she feels powerless.
He/she is in pain.
He/she expects to be confronted/treated with hostility.
He/she has been in conflict with the individual in the past.
He/she feels justified in being angry.
The site warns against speaking to the potential attacker in a condescending tone, or using jargon. But again, if you suspect something’s wrong, it probably is. Gun or no gun, doon’t wait around to find out. [h/t GeoffB]