Working as a bartender, when I was a 20-something, with no formal firearms training and zero handgun experience (although I did have a subscription to Soldier of Fortune magazine) and very little impulse control, a guy with a bloody chef's knife tried to come behind the bar. We had an S&W in a cigar box, under the register. By the time I had the gun in hand, the guy with the knife was very close to me, maybe 6 or 8 feet away. Holding the revolver (I think it was probably a Model 36), with both hands, front sight right between his eyes, "Take one more step, and you're a dead man." He froze. "Put down the knife". He did. Everyone else, in the bar cheered and clapped!
As he turned and started to leave, 2 more guys came running in, one of them dropping the first guy, with a flying drop kick (ala All-Star Wrestling). Then the police showed up. Turns out, the first guy had pulled the knife on the other 2, trying to rob them. They managed to turn the knife on him. It was his own blood on the knife, and he was trying to get away from them, seeking refuge in the bar.
When all the dust settled, my boss asked me why I didn't just shoot the guy. After all, he had a huge knife, and was well within its striking distance. It very likely it would have been seen as a justifiable shoot, but I thank God every day that I didn't pull the trigger. To be clear, if dude had made even the slightest move, in my direction, I was ready to disconnect his brain stem.
I like to think that, maybe, the knife-guy re-evaluated his life choices, and became a model citizen. At the vey least, he was afforded a second chance.