Currently the standard is that a reasonable person would agree. I can't fault that.
For a reasonable person, "threatening" covers a range: anything from "mildly troubling" to "immediate, inescapable threat to life if you don't take action this second", with increasing levels of threateningness warranting increasing level of response.
You suggested something different: an on/off switch where we go from "that guy's gun is none of your business" to "that guy is so threatening you need to shoot him right now" with no transition in between.
So where's YOUR line? The reasonable person test doesn't help us answer this question if you aren't being reasonable.
For instance, as a test case, take a scenario that once actually happened to me:
I walk into the restroom of a Burger King around midnight on a Saturday night. There's a large man standing at the sink. As I'm doing my business at the urinal, he starts to chat with me. He turns and I see that he's making a show of trimming his fingernails with a 12" Bowie knife. He's between me and the door so that I'll have to brush up against him if I want to leave. The restroom is small - everywhere in it is within one step of knife range if he chose to start stabbing or swinging.
Which do you think would have been appropriate in this situation?
- I should have minded my own business and not cared at all that I'm in a confined space with a guy who's armed and much larger than me, as trapped as he chooses to make me. Any concern is unfounded.
- I should've (if I had the presence of mind to have a handgun with me) shot him.