@asdgfh I wasn't gonna bother with this manga, but what you said had me curious. Looking at chapters 3-4, my layman's guess is that in many US jurisdictions you could make a fairly solid case for self-defense on the part of the MC. However, the author mentioned several "conditions" that would need to be met for Japan's legal system to make that consideration. The truth is that the situation is similar here in the states as well.
Stateside, I think you'd need all 3 of these things at bare minimum to be true (ultra simplified):
Opportunity - You were able to be harmed
Intent - There was an intent to harm you
Jeopardy - You were in actual danger of being harmed
There's much more to it.
This article seems to be fairly consistent with my findings. It's worth a read.
Even in the US, affirming self-defense is a messy process highly driven by emotion. Things you say, things you thought you did vs things you actually did vs things someone else thought you did - They all come into play, usually against you. You might get a crappy attorney, a vindictive prosecutor, a prejudiced judge, an ignorant jury, and an insufficient evidentiary/discovery process. You might also be wrong, despite what you think. I'm not getting a lot of information on Japan in English other than some vague notions similar to what I described above, hence why I'm citing US law. The Diet does seem to recognize that self-defense is justified in cases of imminent harm to yourself, and reasonable force seems to be a real sticking point.
In chapter 3, we see the MC take a knife from an assailant who already demonstrated intent and opportunity by grievously harming a nearby colleague. The MC then fled (retreated), was then pursued by the assailant, and became jeopardized when the assailant brandished a second knife and begun to attack him. The MC then swung the knife in his own hand and stabbed the assailant in the stomach - the assailant ceased the attack and desisted, fleeing (escaping) the scene. The MC did not pursue, chase, or further antagonize the assailant.
It looks good on paper, save for the fact that the MC caused grievous bodily harm. He did not unnecessarily antagonize the assailant nor did he approach the assailant with intent to fight or escalate. Multiple witnesses present should in theory corroborate his story. The issues we see in ch.4 onward are some of the things that can screw you in the US: at least one witness took a photo of the aftermath that suggested the MC was the assailant. It was the only photograph of the incident thus far, and the strongest evidence. In this case that one witness changed the narrative intentionally or otherwise and that could sink him right there. The other is that even though he was being attacked with a knife by someone who already used it to harm seconds prior, stabbing the assailant may still fail the reasonable force clauses in Japanese law.
Lastly, the MC was not able to speak to the police coherently or accurately, nor was he willing or able to continue his defense through legal counsel. It is highly likely that he botched his own explanations and everything he said will be used against him. The things he didn't say may be seen as admission of guilt. A Japanese judge used to dealing with the guilty (
over 99% conviction rate) could almost certainly convict either way.
So what's the answer? I don't know, but he's probably fucked in court and he's definitely screwed socially for life. With Japan's high suicide rate, the poor kid might not make it no matter what happens. Life ain't fair, gentlemen. Know what you're doing and be strong enough to choose your fate.