Ugh, I mean... he was literally just a little kid? It was a fucking accident. And yeah, he had multiple chances to confess but didn't, that was shitty of him. But his crime is way less worse than what we suspected Makiko had done. It's a little harsh to tell a kid (he's like, 19 here?) who's been guilt ridden all his life that you would "resent him forever". The house burning down fucked them over, sure, but it was still Makiko's meddlings that resulted in almost every event in the story. She's still the worst one here.