One example of betrayal in Lord of the Flies occurs early on when the boys first meet on the island. Piggy reveals his unflattering nickname to Ralph, asking him not to tell the other boys. Later at the first assembly, Jack mockingly calls Piggy 'Fatty' to which Ralph blurts out Piggy's old nickname for all the other boys to hear. Later Piggy confronts Ralph:
"Piggy's classes were misted again--this time with humiliation.
'You told 'em. After what I said.'
His face flushed, his mouth trembled. [...] 'About being called Piggy. I said I didn't care as long as they didn't call me Piggy; an' I said not to tell and then you went an' said straight out---'" (25).
Ralph['s early betrayal of Piggy reveals his immaturity and lack of sensitivity when dealing with relationships. The scene between the two boys also shows Ralph to be earnest and practical when he reminds Piggy that Jack's nickname was much more insulting.
In my opinion, the most notable act of betrayal throughout the novel is when the majority of the boys choose to join Jack's tribe at the other end of the island. In Chapter 8, Jack fails at his attempt to usurp power and leaves Ralph's group. Shortly after, the majority of the boys sneak away to join Jack's tribe while they are in the middle of collecting wood for the fire. Ralph feels utterly betrayed by the boys because he was democratically elected to be their leader. Ralph is deeply troubled by the fact that the boys joined Jack's tribe and contemplates giving up. Jack's decision to leave the group and the boys' deciding to join his tribe are considered acts of betrayal. At the beginning of the story, they put their trust in Ralph and agree to make him their leader. By leaving his group and following Jack, the majority of the boys on the island betray Ralph.