I have been thinking about this quite a bit. I witnessed a man trying to kill a woman with a large knife in a Johannesburg shopping mall once, and I intervened (by firing a warning shot into an escalator) to make sure the woman wasn't stabbed again. Plenty things I learned that day: not just about tactics, but about myself. Here's what I know:
1) The goblin had a double-edged knife which he held blade down and struck this woman very hard in the root of the neck. I couldn't see where the blade went because it was on the other side of the woman's neck and I was standing sideways of her. All I know is she dropped straight away. If the knife had found its mark it is quite possible the woman would have died whether I intervened or not.
2) I got the goblin's attention which prevented further stabs. I was about 15-20 yards away if I remember correctly but all I had was a Baby Browning. I wanted him stopped but safe direction of fire was scant, and I took the decision to fire a round into a corner escalator. If this happened again I would do things differently.
3) The goblin turned and ran and I ended up chasing him through Hillbrow (about two blocks) before I caught up with him and there was a brief evaluation of both of our situations and he dropped the knife and got taken in by store staff.
On balance I think if it is a frenzied attack by two guys with machetes and they already have the victim down, the outcome is going to be very poor for the victim.
Having said that, I have seen my fair share of machete (or "panga") victims in Johannesburg, and there are those who have got away and survived. One of them was in a similar situation where three or four goblins attacked him with pangas and tried to cut his head off. They hacked the back of his neck so that a wedge of tissue was removed all the way down to the spinous process of C4. When the patient was lying supine on the trolley, I could see the other radiographer's face through the victim's neck.
I have to say on balance in this situation there is not much that could have been done for the soldier. The chances for survival would be slim: you would need a person right there at the scene who already cleared leather before the goblins got out the car and who already had a clear shot and who already had the determination to absolutely and finally stop these men. I think that is a tough ask in any country, not just in England.
However, I have to side with those who criticized the crowd's reaction after the killing. Here you have two goblins bouncing around in front of a camera with the weapons of death still at hand, standing free amongst the innocents, having just killed a man.
What I would like to have seen is a mob encircling these vermin, with whatever tools they could get at hand.
There are stores and cars accessible, if it was me I would be looking for weapons FAST. A wheel spanner, a tyre iron, a fire extinguisher, an old man's walking stick. Something you took from a shop, even a can of beans or an umbrella would have been better than nothing. The disturbing thing to me is the inaction after the man was killed and the threat was still present. I am not saying anyone should have tried to tackle these men straight away, but at least arm yourself with whatever you can get.
That's the nucleus of the debate: the fact that physical defense wasn't even on the radar when these evil men had already demonstrated their intent.
There is one variable of note: and that is I have had my thinking changed about what to do in a situation like that because I have previous experience. I have replayed what happened in Johannesburg hundreds of times and I have discussed the incident on TFL. That's a good thing, plenty things I would change if I could do it over again. I also have an appreciation of what could happen to me if I get it wrong, I have seen the wounds and they make me very cautious.
But the bottom line is, it is very difficult to predict what you will do in a situation you haven't faced before.
When there has been harm to one and if you intervene, such harm could be on you also, your thought processes out there on the scene may be very different from what you think whilst sitting there at your keyboard!