From what I see; the victim's body language is - after the "WTF?" from the female's initial strike (Which to be honest - who wouldn't do?) entirely conciliatory. He's not presenting a 'tough guy' stand - his arms are down, palms outward, shoulders slumped etc. Classic submissive behaviour.
While we're blaming one guy or the other here; let's not lose sight of the true instigator: the female. She was the one that was causing/inflaming this whole thing. Bid dude was following her
lead; even though he might not have believed so himself. Not to justify in any way his actions, but they are
understandable. (To be clear; I mean that not as justification but in the same way that we understand why a mugger will go after a little old lady instead of a seven foot biker.) She
was completely out of control, her 'boyfriend' followed suit.
Quite frankly, the guy was toast the second the big guy walked in - the determination to do violence had already been made.
Now - to the victim's actions: Yes, the attacker's first strike knocked him near the door, but this was no boxing punch, it was a full-power bare knuckle shot to the head. The guy might have been in the open door, but for the second he was there was far too stunned to take advantage of it. No-one
I believe would take a hit like that without getting his jingle jangled.
The victim is clear for approx. a second - though the second strike doesn't come for about 4 seconds after the first; the attacker has a hold of the victim by the right arm, or at least appears to. Also, keep in mind the shot knocked the guy to the door, which pushed the woman, who was blocking the door, outside. IOW, keeping him in. In any event, he does try to move to the door when the attacker strikes again with a hammerhand in the back of the skull. Effectively, lights out at that point.
Now; as to your other point:
What I am wondering though is if we are placed in an "unavoidable incident" as Dave says above, do we do nothing because the guy is decidedly bigger, stronger etc.? Given whatever training we do I mean. Last time I checked (in Aikido) one did not allow the attacker to take one's energy and will to survive via intimidation tactics, and even when it does happen, do we not even make an attempt to survive?
Sorry Larry; I'm not really sure what you mean by this. In the video, the guy had given up simply because there was no fight in him - he was a rag doll after those two massive hits. Aikido-wise; I suppose its possible - highly doubtful, but possible - that a highly experienced practicioner (Say 8th dan or so) could effectively redirect that shot. Undoubtedly could on the mat; but again, we're talking about two different things here. Aikido is one answer - it is not the whole
As for myself and what I would have done, I'd have struck first - attacked full-force high and low - to the head and knee simultaneously. Hardly aiki and likely not effective, but in that situation given the environment the only real chance I'd have. The big guy was in control of the situation - he chose the moment of the attack and held the initiative. Tactically, you have to take the initiative yourself and force the opponent onto the defensive if you're to escape an SD situation and in this case; the only real way to do that is a sudden blast attack. Remember; the guy thinks like a Grizzly bear: "I'm the Biggest Baddest Dude, I can Do What I Want." He's not expecting an attack; especially a paired destabilization/antistructural strike. It's the only shot I'd get, I'd make sure it was the most effective possible.
Is that aiki? No way - the very antithesis. Is it legal? Possibly - it's definitely in that blurry area in which you'll be convinving a judge. But letting a monster like that keep the initiative is like trying to tenkan with a freight train.
Now; from a purely aikido perspective: going frame by frame; it's possible (again assuming a high degree of control and ability - far, far above what I posess) the victim could have at the moment of attack slidestepped into a modified tenkan to avoid the initial strike. Of course; that'd send him directly into the psycho girlfriend, who was blocking the door. From that point, the 'maybe's' start to build exponentially.
End result: A two-on-one attack in which the victim had no real means of escape or avoidance. He was targeted, he was isolated, he was taken out and nothing - save every person in that building jumping on the pair and likely even not that - could have stopped it.