The breakdown on the island is not caused by one individual only, but by a number of factors. However, one can point to a few characters, one specifically, who can be said to have instigated and encouraged the boys' descent into savagery. This character is Jack, who, with his squad of choirboys (excluding others such as Simon and Sam n' Eric), had planted the seeds of ill-discipline and a descent into depravity.
From the outset, one notices Jack's commanding and domineering nature, as well as his savage streak. He is intent on being a hunter and ignores doing the more practical and rational duties, such as building shelters, because they are mundane and boring. He seeks the excitement of the hunt. When he misses killing a pig on his first venture, his blood-thirst is heightened and he becomes insistent on going on a hunt. In this, he inspires many other boys to join him.
Jack and his hunters ignore Ralph and Piggy's consistent appeals to keep a signal fire burning so that they can be rescued and to have the littl'uns taken care of. Their focus becomes twofold, hunting pigs for meat, and finding 'the beast'. They ignore the reality of their situation: that they are on an island and may never be found. Instead, Ralph, Piggy and others who support them become the enemy. Jack sees them as a threat who has to be either obliterated or forcibly detained.
Jack's power is vested in the support he gets from the other boys such as Maurice and Roger (who clearly has an evil streak and is later responsible for Piggy's death). They have tapped into their savage nature and just as much want to see blood. They have become a tribe. They paint their faces to mask their identities. This act encourages them and they become more volatile.
Although Ralph had been chosen as leader, his power over the boys gradually erodes until he is left with only the twins (Sam and Eric), Piggy, Simon and the little ones to support him. Simon's death is an indication of the huge depth of rancour into which the boys had sunk - even Ralph finds pleasure in 'killing the beast'. When the conch is shattered and Piggy killed, the savagery has taken over completely. Ralph has lost his power and is vulnerable. When Sam and Eric are captured, he is at the mercy of the hunters who have now gone full circle and have become completely depraved. They are driven by malice. Order has been replaced by a complete loss of humanity.
Other factors which led to a descent into disorder are fear of the unknown (symbolised by the 'beast'), the lack of parental guidance, naivety, the desire for freedom from rules and regulations, the fact that the boys were immature and did not know how to control their instinctual savagery and of course, their fear of victimisation - Jack essentially bullied them into joining him, and some of them were afraid of missing out on the 'fun'.
Orwell wishes to indicate man's inner depravity. We contain this by creating rules and, by imposing them, maintain order. This is what a well-structured orderly society requires: that we abide by the rules so everyone feels safe.
Orwell illustrates that when these boundaries become muddled or these rules are ignored or abolished, man delves into his most basic instincts. He becomes a savage. The point is that we all are innately sadistic. It is in our nature and, given the right circumstances, such as the loss of order and control, what little goodness we have will dissipate, and our barbarity will flourish.