The adults are the ones who created the problem; they are the ones who bicker and can't get along to the point that a cataclysmic war is started that is the reason the boys are on the plane in the first place.  The dead pilot and later, the dead parachutist, represent the fact that those people let the inner evil beast out and that brought about their deaths as well as the deaths of others.  When Simon sees the pig's head on the stick and he has his "conversation" with it in chapter 8, the description of the head (which becomes, to Simon, the Lord of the Flies), is that it had "...the infinite cynicism of adult life."  This means that it believed that all people are evil and self-serving; that all adults are skeptical and even cynical, about any deeds one does because they know that people don't act out of goodness, but out of evilness.  Golding is implying that as one ages, the evilness inside of each person, comes out more and more.  The boys on the island, though children still, have grown up very quickly because there are no adults living with them.  They have become adults while still boys.  The only living adult that appears in the story is the naval officer who finds the boys at the end of the story.  He, at first, thinks the boys are just playing and then when Ralph cries, he is embarrassed. He is portrayed as being unaware of the evilness but his lack of awareness is due to his desire to be unaware.  He doesn't want to know the truth about what happened on the island because the truth would be ugly and might make him evaluate his own life.  Instead, he looks away.  Golding is showing that adults are the biggest source of evil through both their actions and their inactions.