A question often asked of this novel is whether the boys bring evil to the island with them or whether the island exerts an evil influence on them. This harks back to the classic "original sin" debate, and it's sufficiently interesting to warrant some exploration. I'll attempt to look at both sides of the argument, and give my own conclusion; yours may differ.
One way in which the boys clearly do bring their own destructive evil to the island is symbolically, through the creation of a "long, painful scar" through the jungle, brought about by their crashing 'plane. This demonstrates that the damage caused by humans is inevitable, in Golding's opinion, and certainly suggests that "mankind's essential illness" is brought with them.
On the other hand, there is evidence that evil was previously present on the island, in the sinistersymbols encountered early in the novel. For example, the birds that rise with a "witch-like cry" suggest that the same inevitable immorality present in the boys finds its manifestation in the other forms of life on the island.
The boys themselves believe that the great expression of evil in the novel, the Beastie, was there before them. However, the "snake-like clasp" that Ralph discards soon after their arrival could be seen as a metaphor for the attachment of evil to all the boys, and Ralph himself admits that if the Beastie were real, it "leaves no tracks" - this echoes the comment (from a source who currently eludes my memory) that "if there were no god, it would be necessary to invent him." Likewise, the Beastie can have no influence without the fear of the boys to exploit.
A furthur way in which the boys bring evil with them is in their exploitation of the already-present wildlife. The boars, for example, have survived in harmony on the island for years - but suddenly, with the introduction of the boys, they are hunted down and violently slaughtered. While the hunting in itself is not necessarily to be condemned, Golding display's his distaste for the style of their assault by attributing the first successful pursuit with a style strongly reminiscint of rape, as the boys enjoy "taking away [the boar's] life like a long satisfying drink." There can be no doubt that the boys bring this sadistic pleasure with them, as the boards pre-date them in habitation of the island.
There are also on the island natural structures to which the boys attach a military purpose. Like the boars, the "rock fortress" had been a permanent feature of the island, but it is only the human interpretation of these passive structures that allows them be to used a tool for the murder of Piggy with another natural phenomenon, the rocks. In the abscence of the boys, there is nothing sinister in these rocks - but once they arrive, there is evil to be found in them. Humans, suggests Golding, inevitably find weapons to hand, in any surrounding, however idyllic.
In conclusion, Golding definately appears to support the idea of "mankind's essential illness," a theory he first expresses through the spiritually-astute charcter of Simon, although that character's youth prevents him fully explicating his intuition. The islands provides an arena for their evil, but this does not serve to exhonerate their boys - just as modern disgust is levelled at Roman gladiators, not the arenas in which they fought; these are treated as great relics. Indeed, the initially paradisical descriptions of the island suggest that the boys would have found evil in any setting, and that description is only occasionally punctuated by implications of any already-present evil. The boys themselves provide the mechanism for the potential in the isalnd to be realised as evil - the island itself is a vehicle. According to Golding, the evil is brought to the island by the boys themselves.