Kind words appreciated, 3000.
As you may have noticed scanning this corner of GATE, I try to stay away from "scenario what-if" questions. They tend to branch off into countless "but what if THIS element changes" rabbit trails, and are much better suited for a thread in either Tactics and Training or Carry Issues, where lots of different folks can chime in. GATE is geared more to questions suitable to short answers by a single poster.
Where that keys into it is going to be the question of, "How do we KNOW he's going to kill someone if we don't shoot him down?" And the questions of, "How do we know he's alone or not," and "What's the angle of fire," and...well, you get the drift.
That said, I think your core question goes to the issue of "OK, I decided to carry the convenient little low power gun that is very difficult to shoot well under stress with its small size, tiny grip, vestigial sights, and low powered ammunition...and now I may have to make a precision shot with a round powerful enough to instantly incapacitate, and I may have the wrong tool for the task."
Personally, answering your question after a couple of days of carrying a Glock 17 I've won matches with and is loaded with 127 grain +P+, I'm very happy to have accepted the minimal inconvenience of carrying that with a blazer to cover it yesterday when teaching at the college, and a light vest to cover it while having a more casual day today. The blazer yesterday and the vest today carried my cell phones and such that I needed to have on my person anyway, and were more convenient than inconvenient to wear.
3000 fps, I think one of the lessons you've put on the table in front of all of us here at Glock Talk is that we carry the damn thing in case we REALLY need it to save our lives and the lives of others, and that carrying something that may not be up to the task in the name of convenience is something we need to constantly consider and reconsider.
And I thank you for that.
I would really suggest you repeat your question in Carry Issues or Tactics and Training (feel free to link to this discussion between you and I in your opening post), and "call in the Glock Talk Brain Trust."