I would argue that all the boys on the island are responsible for Simon's death. If we want to assign levels of guilt, Jack and Roger would probably be at the top. They demand respect as hunters, they withhold food to drive the others to the point when they cannot say no, & they incite the mob that eventually gets Simon killed. They want everyone chanting and dancing, because that inspires chaos, & chaos works perfectly for them. But all the other boys have a hand in it too, including Piggy and Ralph. The description of Simon's death mentions "the crowd", meaning they were all working as a mob.
The sticks fell and the mouth of the new circle crunched and screamed. The beast was on its knees in the center, its arms folded over its face. It was crying out against the abominable noise something about a body on the hill. The beast struggled forward, broke the ring and fell over the steep edge of the rock to the sand by the water. At once the crowd surged after it, poured down the rock, leapt on to the beast, screamed, struck, bit, tore. There were no words, and no movements but the tearing of teeth and claws.
Thus, everyone present had a hand in attacking Simon, & preventing his escape. But Piggy doesn't want to hear this. He denies their crime, & blames everything and everyone but Ralph and himself.
"It was dark. There was that--that bloody dance. There was lightning and thunder and rain. We was scared!"
"I wasn't scared," said Ralph slowly, "I was--I don't know what I was."
"We was scared!" said Piggy excitedly. "Anything might have happened. It wasn't--what you said."
So Piggy insists that it was the darkness and the storm that caused Simon's death. He blames their actions on their fear, trying to rationalize their crime through their emotions. He ends his argument by claiming it was an accident.
"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."
Indeed it was an accident, but most of those boys probably knew on some level that they were attacking a living, breathing human, & not the beast. They knew, but they couldn't stop themselves.
In the novel "Lord of the Flies" the boys are stranded on an island without adult supervision. Their animal nature soon takes over as their civilized habits fall away. Simon is a representation of a Christ like figure. He is brave in his own way. He faces the beastie so that he can learn the truth about it. Jack and his hoard of boys become more and more savage.
To the boys Simon is odd and they do not understand him. He is a direct contrast to Jack's emerging evil side. He is sensitive and meditates. He is good and looks out for the little children.
Fear takes over the hoard of boys . They become like a crazed mob. They chant to kill the beast while dancing in circles. They are in a mad state and Simon tries to tell them there is no beast. They long for the spilling of blood. Their feelings and need intensify. Simon is in the center of the boys. Simon tries to stop the boys behavior when he falls. The boys go after him. They beat him to death savagely. Then they leave him bleeding in the sand. The water dresses Simon's hair with brightness symbolic to a halo. Simon's body is carried out in the ocean. (53 &54)