I mean try to look at this from the perspective of someone who wasn't there and isn't reading a manga of the events. There's a picture of a guy with a knife, and another one at his feet, stabbed. There's another guy in the hospital stabbed. There is really only one conclusion you could draw from this evidence alone that makes sense, especially because normal ordinary citizens don't just have large kitchen knives with them just in case for self defense.
As far as eyewitnesses goes, there's probably as small amount that there there from the beginning, and a larger amount that arrived just to see mc brandishing a knife while chasing after the hooded guy and you can imagine how that would look. And thmen the eyewitness who to them it seems like two people having a knife fight over a girl, and then there are also eyewitnesses that arrived only at the end to see mc brandishing a knife with two people stabbed.
As for his trial, Japan is kinda famous for having an extremely high conviction rate so his chances aren't looking good there. Though they didn't seem to take him in for the stabbing of Arata, and like they said with the other guy missing there isn't really any case. And self defense laws really can be tricky, even in the US. I won't pretend to know how they are in Japan though. There's a good reason that lawyers always say never to speak to the police, and that's because attempting to be your own lawyer based on what you think makes sense, rather than what the actual law is, or without consideration how outsiders will view the case, is a folly.