Ok, Ive got the gist of this: There are two major humanly created things going on:
Komachi is the serial killer, trying to get rid of Shiina and Seo (It was Komachi that lead Seo to the store room). Her change in attitude was due to the fact she did not die. (1)
There is essentially a time police involved, starring council pres guy. He too apparently sees these corpses but chooses not to deal with them, as they change the future unpredictably. Or rather, that must be his guess since he shouldnt have done it in the first place.
(1) Is strengthened by the fact that the first chapter stated in overdramatic fashion: Komachi should have died. Indeed, it is noticeable that each time a "major" corpse gets erased, seemingly parts of character personality get stripped away. E.g. the stalker girl becomes literally obsessed, Satsuki being more and more obsessed with saving everyone. I think the author is gonna pull a "Steins gate" where Satsuki must undo her changes at some point since this will end in disaster otherwise, by killing necessary people herself.
I dont think its a coincidence either that Komachi had to die twice consecutively, that was the world trying to fix itself.
I think the president is trying to save the world right now, rather than being an evil antagonist.
Lastly, according to the story, even before this manga started, MC saved a lot of people - successfully. Otherwise people would believe her by now. Why then has there been no complications before?
It'd be very straightforward for Komachi to be the serial killer... there's bodies everywhere now not because Satsuki is saving people, but because she happened to save a psycho murderer. And she seems so sweet and kind, it makes sense she could be indirectly inspiring things too behind the scenes: feeding Michiru's obsession, poking at the class president's insecurities in just the right ways.
But some of these deaths really do not seem to plausibly be murders. Like I guess it's possssssiiiiiible she greased up the gymnastics equipment, but I don't think the soccer ball through the window could be anything but an accident. (Though that one had the whole weird Satsuki death scene that I can't really figure out.) So while I do think the class president is trying to avoid some kind of cascading butterfly effect or something, I don't think it's because people's personalities get ore extreme.
I also just reaaaaaallly suspect that blonde teacher guy. She goes to him and talks about the murder methods, in detail, the day before each murder!