It's not the number of attacks here. It's the way he does it. He doesn't hit him like people usually do. He goes in disregarding the attacks and throws him on the ground. Finishes with a puff and walks off.
He could have just pushed him down and walked off.
It's rather obvious that he was doing it as a show of his strenght to stop the kid from messing with him further/revenge from past.
WTF

........ where are you getting all these little details like "Finishes with a puff" i see and hear no indication of a "puff"
"It's rather obvious that he was doing it as a show of his strenght to stop the kid from messing with him further/revenge from past" ....... I strongly suspect ONLY in your dubious opinion. Once again i see no evidence to support this idea, in fact much to the contrary. The QFK spends no time gloating over his victory, doesn't taunt or even admonish the thin kid, but promptly walks away saying nothing ...... odd behaviour for someone "showing off his strength".

Serious question do you have some issue with violence in general?? Buddhist, pacifist, etc.
Am baffled at your attempts to bend this situation around 180 degrees.
The thin kid attacks the QFK strikes him a good hard shot to the face that almost bounces his head off the wall !!!.
Lands a few other more minor blows.
The QFK takes all this with no offensive action ....... probably hoping thin kid tires of it and leaves him alone.
More blows and QFK defends himself, attacks once and leaves. How is this anything other than TEXT BOOK self defence.
You keep talking about the risk to the thin kid ( which many would reasonably argue he generated himself ), but seem oblivious to the risks that the QFK was placed under by an apparently unprovoked attack.
What if the thin kids next attack had been a straight kick to the knee ( quite capable of destroying the joint and imposing lifelong dysfunction )
or the next heavy punch hit the spot just above and behind the ear ( which can render anyone instantly unconscious with very little force )
Seems your concern is that the QFK WON by using violence .......... but this totally ignores the main fact that he has attacked 1st and merely defended himself. I find that pretty twisted logic. How about some concern for the damage the ATTACKER may have done.