This. I read this theory on Twitter and it makes the most sense out of all the other ones.
When I first started reading this manga, I never thought that the ending would be so sentimental. I wonder if there were any foreshadowing in the previous chapters though. I doubt they decided to just kill him off. I'm too lazy to reread the manga but if anyone does and finds anything else other than the last chapter to back up this theory, that'd be nice.
Gosh im so sad ╥﹏╥ hes leaving and hes gonna die soon cuz of that. Seeing his face he was about to cry because everyone loved him so much thats why he coverd his face with cream!! Ima cry i really love Sakamoto, so sad that hes leaving..
While this is only a theory I think it makes sense. At the beginning of the last chapter, Sakamoto is in a closet, pretending to be a corpse. Kubota-kun is talking about how he doesn't know anything about Sakamoto and that he will now gradually get to know him. This never happens. Sakamoto says that he's going overseas or takes part in a Mars project or something. He talks to 8823-senpai who says that it's a lie. Then Sakamoto says that the only thing he regrets is not counting the holes in the ceiling of the inpharmacy room. This implicates that he has been in the inpharmacy room very very often... It's because Sakamoto is sick. He has a terminal illness and he wanted to make the most out of his last high school year or in general last year in his life. Yes, it's just a theory and everyone can have their own conclusion but I think it really makes sense like this.