Original sin, or ancestral sin, is the sin inherited by all of humanity from Adam and Eve after their disobedience to God in eating from the Tree of Knowledge - this is commonly referred to as The Fall of Man.
Golding disassociated himself and the novel with the theological side of original sin; rather, he simply believed humanity was inherently evil. As he once said about the novel, 'terrible things' are being done 'by children to children' and 'things are the way they are on the island because of the boys', not because of some stroke of bad luck.
The boys, including the tribe, are terrified by an evil which they perceive to be around them, an evil which becomes a physical, tangible one after the body of the dead parachutist is mistaken for the beast.
It's not until Simon's imaginary conversation with the pig's head, that we realise what Golding was perhaps suggesting: the notion that evil exists within us all.
The English decorum and respect very quickly faded once the control of society was removed.
Even the smaller boys see evil manifest itself; the unthinkable acts committed, especially by Roger, point towards a heart of darkness and not towards Rousseau's noble savage.