it wasn't that it was the MC's fault but moreso that the MC allowed him the peace needed to finally move on. nageki took his own life and his spirit was trapped, unable to move on. the MC just treated him as a regular birdie, allowed him to live through the days as if he was actually in their class, and he fell in love with them. that feeling of love allowed him to remember his past and he was able to move on.

it wasn't the MC's fault that nageki died since he had already been dead, but it was thanks to MC he was able to peacefully move on.

Thank you so much <3
Now I understand =>

Agreed. I find Nageki's ending disappointing as well
I would've been perfectly fine with it being a normal (or alternative) ending, but he could've been revived for the good ending. I mean, stranger things have happened in the game. If Nageki had "passed on" only to return as a (normal, alive) human the next day just because his love for you/Hiyoko was so sincere and powerful that it granted him a wish, I wouldn't even question it.

If it had a happy ending it wouldn't hit you in the feels with quite the impact. Try to appreciate it for what it is. It's supposed to be tragic. The fact that it's evoked strong emotions is a sign of good writing.