According to Golding, it is clearly the forces of evil within us all that will triumph when the trappings of civilization are taken away. His portrayal of the way that Jack and his hunters triumph over the representations of civil order (Ralph) and science (Piggy) is his way of suggesting that when the strictures of society and civilization are taken away humankind will resort to selfishness, evil and violence to enforce those emotions. Even Simon, frequently labeled as a Christ-figure, cannot overcome the forces of evil and instead is murdered by the boys in a fear-induced frenzy.
Certainly the ongoing war in the outside world contributes to the general sense of evil being the normal state for humankind.
Considering the historical context within which Golding composed the story, it would make sense that his outlook was dark. Having witnessed the horrors of World War II and the beginning of the Cold War, he had every reason to be critical of the human race's capacity for good.