This is a stupid as hell arc. First, it goes with a really crappy writing style where we literally see Shiki's inner thoughts say "You already know that I hurt Yagami on purpose". There's no way that the story actually goes the "I did actually hurt a close friend on purpose for personal gain" route. It'll be some kind of reason that makes Shiki look at least slightly better than that. And that's the thing. Shiki knows his own reason. And then we can literally see his inner thoughts. And yet the author just makes him only think about this sentence even though that makes no sense? That's such crap writing. If the author actually does make Shiki selfish who hurt Yagami on purpose, then I'll retract this because that could be good writing. Sometimes you do have to face the fact that people close to you are jerk asses. That could be a good lesson. I just HIGHLY doubt it'll go that path.
Second, the entire premise of this arc is, in fact, stupid. Yes, Dai could just move on with his life. He could maintain an interest in kendo and just continue in college AFTER he raises his grades and graduates high school. No, Nagi doesn't need to violate Dai's privacy and EXPLICIT wishes of wanting to leave it alone. Yes, some traumas are best confronted. This is a generally okay trauma to just leave alone, though. The author will obviously make this turn out for the better because everything is in the author's control, but in real life, this could easily just be something you leave behind and move on from.