Kouichi killing Reiko Mikami. It's worse since she is begging not to die, as well as the fact that because she is The Extra; no one will remember her.
Akazawa being impaled on glass and dying, while finally remembering her Forgotten First Meeting with Kouichi. Although her chastising Kouichi for being insensitive while she's dying almost adds a note of humor.
Let's be honest here, the entire series is tearjerker material. Start to finish. This is a story where a curse is hunting a group of middle school children and completely gets away with slaughtering almost the entire class in really painful, gory, horrifying ways. The students are well aware of this and the curse doesn't kill most of them until the last couple episodes. On the way there, we see the student body going insane from the tragedy and spilling many tears over the deaths of their friends, but by the end of the series they've all turned on each other and start dropping like flies while the other students barely even give a shit anymore. The curse goes after the surrounding adults too - their teacher kills himself in class after murdering his own mother, one student gets killed with her parents driving off a cliff, another student's brother gets killed and finally, the main character has to murder his aunt just to stop the curse AFTER it's already gotten away with butchering almost everyone - but that's okay because it turns out she was already dead after being murdered a year and a half ago by someone who also got away with it. Even at the cemetery where presumably all these students are buried, there's almost no dramatic justification or shits given about them. Finally, it's known that the curse is just going to come back again next year to do it all over again. It was pretty much all for nothing and none of them deserved it. Did you get all that? Here's to hoping the sequel will provide better closure and something to justify all that.
