Who is to blame for Simons death and why? how does Piggy respond and who does he blame?

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.