Everyone is talking about how the shitty teacher shouldn't have been released, but that wasn't how I interpreted that scene. The notion that he got out on parole and immediately tracked down the kid he abused, not to get revenge or do something awful, but to calmly watch him perform, seems improbable. Also they refuse to show the guy's face, even though it was shown a lot previously. I don't think the teacher was actually there. I think Yuu saw someone who looked like him, and it freaked him out. Speaking from experience, when someone does something horrible to you, sometimes just seeing someone who looks vaguely like them can set you off, since you think they've come back to hurt you again. This can occur regardless of whether it is rational for the person to be there. This is most common in the months after the incident, but even years later that fear can come back in some horribly unexpected ways. Especially if you haven't gotten proper psychiatric treatment, which it doesn't seem like Yuu got. I can't prove that it wasn't him, but that was how I saw that scene.
@talltales I more or less agree. I don't think the author was deliberately trying to fetishize the incident, but I do think they got really graphic in a way that wasn't necessary for the story. The sound effects in particular felt really gratuitous. The content warning helped a bit (thanks translators), but people reading the raws didn't get that, and even with the warning it was a really upsetting scene.