Yeah the 'quiet fat kid' = QFK as i like to call him in this vid expresses the very definition of reasonable self defence.
Obviously this is precisely defined in slightly different ways in different countries, but here in the UK ( i know this as am 45 year old life long martial artist who used to instruct street self defence classes ).
Legal self defence is defined primarily by two components:
A: Reasonable fear of attack, if someone has already demonstrated physically or verbally an intention to assault you
then you have a clear right of self defence. ie attacked in a bar then attacker states "am gonna cut you up" if you then hit the guy with a chair, under Brit law your pretty clear, even though you saw no knife. Same situation and you hit him with chair because you THINK he has a knife = thin ice.
B: 2nd principal balances your allowable response to a threat forcing proportionality. Basically it states that your response to threat needs to proportionate to the continued level of threat. So if a guy attacks you with a knife its fine hit him with a chair or a bat. If he keeps coming its ok to hit him again. However as soon as he is unable to present a threat ( unconscious or disabled ) or stops attacking, your right self defence stops dead. So if you hit him and he falls unconscious and you then stomp on his head until you reach the pavement ........ the feds are gonna have a LOT of awkward questions for you ending in you being charged with assault ( or maybe worse ).
Back to the vid. The QFK attempts to defend himself non offensively by blowing the blows.
this fails and he gets him a few times. For me this establishes that the attack genuinely intends him harm.
The QFK then takes offensive action, and makes only ONE attack, he makes no attempt to follow the attack up and actually immediately leaves the area. Once the threat stopped he stopped too.
For me this perfectly defines a balanced, reasonable reaction to a self defence situation.
IMHO once you initiate an unprovoked attack on a another you open a Pandora's box and really have little right to bitch about what comes out of it to meet you. Within the bounds of the law you deserve whatever you get.
I'd also like to say that in my experience ( i grew up the only black kid and book worm on an all white rough English housing estate in the 70's, with white foster parents, had to fight my share to not be bullied, and watched lots of others get bullied to fuck ) if more kids and people generally actively defended themselves bullies and other social predators would be less inclined to attack as often as they do. Bullies and social predators aren't looking for a challenge or a fight what they look for is weakness that they can exploit, show them that attacking you WILL have consequences and they will often find easier prey. May cost you a lump or two here or there and its usually not the best response if weapons are involved, but overall being prepared to defend yourself is an essential ( and often maligned ) basic mammalian trait.