'Lord Of The Flies' is based on the William Golding 1954 novel of the same name, which was also made into films in 1963 and 1990. The story tells of a group of school boys who are marooned on a tropical island, and who gradually descend into tribal savagery. This is an energetic type of song, whose lyrics glorify the animal nature that is inherent inside all people. This theme is also dominant in the story told in 'The Edge Of Darkness', where a man, dwelling in the depths of a jungle and freed of all social constraints turns to unspeakable savagery.
In his novel, Golding's intention was clearly to take the counterpoint of previous similar stories where people stranded on a desert island cooperated in order to recreate civilisation as they knew it. In this more realistic story, we see children commonly (and often wrongly!) assumed to represent innocence and fairness return to the tribal savagery of our ancestors, and destroying all trace of civility that some where trying to preserve. As Golding was a teacher, we can assume safely that he'd observed the behaviour of children in the schoolyards and drawn the right conclusions: the lack of social constraints that are ingrained in adults through their upbringing and education is clearly obvious in the way younger children establish their relationships to others. A kindergarten yard is basically a jungle where the strongest simply try to crush the weakest. Take away the discipline that the adult society enforces and you end up with a bunch of savage little animals who "just want to live [their] own fantasy." (Disclaimer: I'm not saying that all kids are systematically either bloodthirsty animals or hapless preys of those, as it also depends on each individual's personality a non-negligible human trait.)
Each character of the novel represents a part of human nature that is either kept at bay or put forward by society in order to make social life as peaceful as possible. The song sees the story most probably through the eyes of the character of Jack, who represents evil and violence, the dark side of human nature. Originally a choirboy, he becomes the "chief" of the "tribe" and throws away all social conventions that make a civilised society possible "Who cares now what's right or wrong [...] We don't need a code of morality." Ralph and Piggy, respectively standing for civilisation and fragile intelligence, try to oppose him and make sure that everyone is sheltered and fed properly, but Jack is only interested in the hunt not for food, but for the thrill of the action itself ("I've found that I like this living in danger").
"Lord of the flies" is the literal translation of the Greek word Beelzebub, a term used for the Judeo-Christian notion of Satan, or evil personified. What the novel highlights, as well as does the song, is that this evil resides in all of us and, provided that the social barriers cease to exist for whatever reason, breaks loose in extreme situations "What was meant to be is now happening." This "Something willing us to be lord of the flies" is simply ourselves.
