The game written by a man who hates the mentally ill, for people who hate the mentally ill.
Saya no Uta sets up Fuminori as a mentally ill man on the verge of suicide, all of his senses being inversed so that pretty is ugly, ugly is pretty, tasty is disgusting, and disgusting is tasty. Naturally, the entirety of where he lives, and even society itself is inherently opposed to what he experiences and so he starts to become more and more of a social recluse in order to avoid having to deal with these things as much. Instead of the game trying to portray him as a sympathetic character going through a situation anyone else would've killed themselves in, including (you), it constantly tries to make him out to be this huge asshole if not outright demonizing him while siding with his torturers.
It first starts with Fuminori trying to cut off his friends, saying he no longer likes them and isn't interested in hanging out anymore, he's blunt and to the point which while not a nice thing, is very understandable and definitely not a bad thing. The "friends" in question then proceed to break into his house in order to "teach him a lesson" with the game hardly able to control its excitement about punishing Fuminori. The "friend" ends up getting killed in self defense by Fuminori's girlfriend (who didn't even know who the "friend" was) and then proceeded to try and frame the defense against a home invader in the most negative way possible to show how Saya is a horrible person for defending her home and how Fuminori is just as bad for... not being anywhere near the situation and not even realizing the aftermath of what happened due to his illness.. but he didn't want to be friends with those people anymore so FUCK him.
Past this point the game tries to switch perspective over to his previous "friends" to try and heroize them in the search of the "murdered" friend who broke into Fuminori's house. Koji goes as far as to start stalking Fuminori in order to pin the murder of the friend in question on him and doesn't stop even when confronted and Fuminori tells him to back off, all while the game goes on and on about how terrorizing Fuminori is a good thing and he deserves it for being a sick mentally ill fuck. The stalking goes so far that Koji and a few others try to illegally access Fuminori's medical records to use his illness against him, of course once again this is portrayed as a good thing that Fuminori deserves to have happen to him.
Fuminori, fearing for his life and freedom, ends up pushing Koji into a well and leaving him for dead. While much less cut and dry legally, it is still definitely in self preservation depending that Koji was quite literally trying to ruin his life on a whim. Koji eventually escapes and goes on an unhinged murderous rampage where he breaks into Fuminori's house to try and kill him just to find out he isn't there, and when he does find out where he is he repeats it and Fuminori is forced to defend himself yet again despite the game trying to paint him as the aggressor.
Gen Urobuchi is a hack who can't write anything other than gay porn worth a damn, but despite all the attempts to warp the perspective of what's actually happening in the game, it unintentionally manages to be a pretty decent story about Fuminori and Saya's relationship and how they come to depend on each other, I guess if you put a blindfold on and keep on throwing darts at a dart board, one is eventually able to land semi-close to a bullseye, which is the best Gen Urobuchi will ever get.