No individual or distinct group out of all the boys on the island were directly responsible for Simon's death; this is ironically revealed in the way that Ralph and Piggy attempt to rationalize and excuse their behavior, knowing that they shared in the murder simply by being present for it.
A significant aspect of the murder is Golding's narration from a non-omniscient, subjective point of view; while we are meant to understand that it is Simon who has stumbled upon the group, Golding has just spent some effort conveying the emotional turmoil of their dance and the brewing thunderstorm when he arrives, and continues with this emotionally-charged interpretation;
A thing was crawling out of the forest. It came darkly, uncertainly. The shrill screaming that rose before the beast was like a pain. The beast stumbled into the horseshoe.
Golding does specify that it is Simon, but most of the description of the scene calls him "the beast". Through this, we are meant to understand that the boys actually think, either literally or through the savage haze of their delirious dancing, that they really are killing the Beast.
Another key element is the fact that we never see anyone but Ralph, Piggy, and Samneric struggling with the implications of what they've done, nor do they confront Jack or the others about it. This emphasizes the depersonalization taking place among the group and leads us to believe that only these "civilized" characters truly realize what has taken place.
Piggy, being the most bound to civilized life, is predictably the most horrified when he realizes what has happened, and he gives all manner of excuses, going to the point of blaming Simon for his own death;
“It was an accident,” said Piggy suddenly, “that’s what it was. An accident.” His voice shrilled again. “Coming in the dark—he hadn’t no business crawling like that out of the dark. He was batty. He asked for it.” He gesticulated widely again. “It was an accident.”
However, it's clear from his frantic efforts that Piggy knows this was no accident, and fears being caught up in the downward spiral taking place.
Therefore, we can argue that everyone on the island, or at least everyone who was present at the dance, is responsible for Simon's death in some way, even if by not making an effort to stop it.