hmmmmmmmmmmmm

well now we know sora doesnt have a good relationship with his family so in some way it probably is related to that but since we dont even know them it would be pretty anticlimatic if one of his family killed him so i dont think it would end like that. But its also worth pointing out how they didnt notice he literally went missing?????

But perhaps he doesnt actually have a family. but someones gotta be paying his school bills and art equipment so maybe he only has someone thats a guardian. also Sora not having an actual family supports how he casually went to the mountains far away.

If we follow this theory, that guardian could possibly be someone like the professor. someone who is passionate about art hence will willingly contribute to soras intention to die after his last painting. or maybe he has manipulated sora into being this possibly suicidal artist(the person killing hinata had a huge smile so its safe to say these murders dont soley benefit soras wish to die after this painting)
-aside from the being his guardian theory, the professor is pretty suspicious rn since he was not seen at the funeral, was present at school right before tokiwa's fall and was not seen at the sketch convention while they tried to find president hase.

soras painting
In chapter 1, a larger panel with bolded words(insinuating what hes saying is important) has sora telling our mc that "art is about telling people what is in your heart, hinata. never forget the message you want to convey" and in a smaller panel a few pages before he says "what are you trying to convey with this butterfly". From this we understand sora has a liking towards symbolism and being able to convey a message from the heart. shinohara senpai is also shocked while looking at his painting and is surprised "he indeed did it" and how "he actually'd pull something like this". As far as we know looking at his painting, it consists of an angel holding an apple(and theres something like a rod in the corner too but im not sure). Shinohara indeed says that he utilised motif and chiaroscuro whereby its presence is obvious as our eyes r directed to the salient apple. Despite the manga being b&w, his influence from 17th century neapolitan art would mean his painting mustve had reddish warmer hues contrasting the black. Sora being high key psychopathic, wanted to enhance the symbolism of the apple that infamously derives from adam and eve. Hinata catches him in a shed deep in the mountains covered and surrounded in what we can only assume is blood(watch it just be paint lol). Quite a stretch but it isnt surprising if sora wanted to compose his piece with as much passion as he could (he seems like a kid who would disect animals anyways. no corpse is found in the shed from what we see but there is a bucket that could fit a smaller animal...hinatas dog????) Back to the motif, alluding to adam and eve, apples are mediums to conveys desire. In chapter 5 we view a side of sora that wishes he had a family like hinata or to even be part of it.
"art is about telling people what is in your heart, hinata". and from the bottom of sora's heart he desired to be noticed and loved the same way hinata was by his family.

but hey, thats just a theory, AGAMETHEORY. ok its 3am im out of brain juice and ready to sleep plus the art is scaring me but i had to keep going back to look at quotes. tbh besides the professor got no idea who would be the killer. unless...ITS KISAKI TETTA(╯°Д °)╯╧╧