What larger, universal truth is presented in Golding's Lord of the Flies through the themes of power, the personal price of conformity, and the monster that lies within human nature?

The universal truth presented by Golding through these themes is a sobering one. It is that mankind, despite what we might like to believe, is not fundamentally good. Rather, we are fundamentally evil, and left totally to our own devices, without some kind of institution to regulate us, we are always subject to reverting to our base instincts, what Sigmund Freud called the "id," that will overcome whatever sense of morality that exists within us. There is much in the novel that supports this pessimistic view.
As for the theme of power, we can see that as the novel goes on, the basis for power is contested and indeed changes over time. At first, the boys seek to replicate, at least somewhat, the order that existed at their school. Piggy and Ralph, in their attempts to establish this order, base their efforts on reason and intellect, which Piggy, in many ways, represents. The conch, for example, becomes an emblem of power, one which summons the boys to the beach. When asked where the "man" who blew the "trumpet" had gone, Ralph answers that "there's no man with a trumpet. Only me" (25). Later, he lifts the conch to assert his qualifications to be leader, saying "seems to me we ought to have a chief to decide things" (28). Jack, on the other hand, bases his authority on his leadership of the chorus, which gives him, as Golding describes it, a sort of "offhand authority," a point he makes explicit before losing the election for "chief" to Ralph (26). Ralph and Piggy continue to appeal to reason, rationality, and legitimate authority as a basis for power throughout the book, but Jack begins to appeal to baser instincts--his ability to hunt, exemplified by the pig he killed, and his ability as a hunter to protect the "littluns" from the supposed "beast" on the island.
Eventually Jack declares his "independence" from the power represented by the conch, and we see that many of his band, most of whom have not left his side since the days he served as their leader in the choir, continue to follow him more or less blindly. Jack is charismatic and he appeals, like demagogues do, to the baser instincts of the boys, both their fears and the fun of the hunt. Whereas Ralph attempts to maintain order, Jack wants the boys to embrace disorder, and they follow him, conforming despite the fact that his leadership more or less abandons the idea that they will ever be rescued. But his flouting of the rules appeals to some of the other kids, who take to him when he whoops, "[b]ollocks to the rules! We’re strong—we hunt! If there’s a beast, we’ll hunt it down!" (130). This line demonstrates Jack's contempt for authority as well as his appeal to the fears of the other boys, who conform to his vision and to his leadership.
As the book advances, it becomes clear that there is no "beast." In fact, this is Golding's major theme--the "beast" is the inherent evil that lies within people, unleashed in this story by the absence of adults, structure, and rationality. Simon (who is later killed by the boys) actually makes this point fairly early in the book, when the boys are debating the existence of the beast:  "What I mean is. . . maybe it’s only us" (126). As it turns out, he is right. Jack is not protecting the boys from anything, he is in fact representative of the beast itself. The death of Piggy (and Simon) at the hands of Jack and his followers demonstrates the power of evil to overcome rationality, intellect, and human decency.