my theory is that it seems like sora is planning to go out the same way that famous guy that he was talking about during the confrontation with the vice president did. ive noticed in multiple artworks hes depicted on train tracks, which is the same way that artist died. thanks to his "its my last piece, after all." we can kinda guess that  he's aware of his impending death, whether it be by someone else or more likely, by commiting suicide like that artist did. considering how not upset he looked by the idea of that painting being his last, I think its reasonable to assume that the latter may be the case.

in case sora didnt kill himself, theres stuff that clues us into who the murderer may be. we  know that there may be some sort of secret in the painting that alludes to the identity of sora's murderer. or since theres apparently multiple people doing the attacking now, the identity of whoever orchestrated soras murder. with this said, I dont think anyone in the art club did it. whoever killed sora probably put him in the art club wall in order to shift the suspicion onto the club members and off themselves. I think this was done because the club members were so often around soras painting, so it could be assumed that they would find its secret sooner or later, and leak the information of the culprits identity.

finally, with all this considered I think the culprit is someone with a lot to lose, and a good amount of respect to their name. this person was able to get multiple people to kill for them to keep their own identity a secret. so yeah, hakuraku.