w\Who or what is to blame for the complete breakdown on the island? What message is Golding trying to convey?

I think Goldin'g's idea of the "darkness of men's heart" or in this case boys is to blame for the complete breakdown of society on the island. The boys, for the most part, gravitate towards Jack and his desstructive chaos. They rebuke the boys that are constructive and sympathetic like Ralph, Simon, and Piggy.

Simon relates one of the main themes of the book. While the boys argue and speculate over the beast, Simon comments that whatever beast is on the island lies inside themselves: the beast is the darkness within the boys.

Maybe," he said hesitantly, "maybe there is a beast."

"What I mean is.....maybe it's only us."

This metaphor is, of course, too figurative for the boys to understand.