That was definitely a dangerous and potentially deadly attack. If I had witnessed this, I would hope that I could take quick and decisive action to stop it. I say "hope" because that was so vicious and so sudden, that I might have been frozen - not with fear, but with shock - my mind not being able to quickly process what my eyes were seeing!
But yes, assuming I was not frozen in place and was able to take action, I would: draw my weapon, move to the best position I could - that is, possible cover and/or concealment, while at the same time doing a quick check of the angles and area behind my target - while yelling (only once): "stop NOW, or I WILL shoot you!" And I would do all those things simultaneously. Then if he did NOT stop, I would do what I said I would do. At least, that would be my plan.
On the other hand, if he did stop and leave when I yelled out, I would NOT follow him, but would observe how he left (on foot, car, bicycle, etc.) and note the direction he took and report that to the cops. Then I would try to render first aid while making sure that someone had already called 911. All while trying to keep one eye out to make sure he didn't circle back for a second chance.
Now, not to hijack the OP's thread, but I think this question is closely related enough to be relevant here. The question being: if you were the victim here, and you had an LTC and were armed, what would you do? And when and how would you do it?
Admittedly, there were some warning signs that the perp gave off that the victim missed that would have told the victim that something was about to happen. But I think all of us, even when we are trying to be observant, sometimes lapse into our own little world occasionally, even if only momentarily. Or I know I do, at least.
So what would you do if you were the victim here? When do you draw your weapon (assuming that you do)? When do you fire your weapon (assuming you do)? If you had spotted the guy giving you the eyeball before he took any other action, what you have done to stop this before it started?
Stuff that we all need to think about. Brings to mind the old saying: "your feet can't go where your mind hasn't already been."