The strongest emotion that Jack feels in Lord of the Flies is the same emotion that leads him to try to kill Ralph in the end of the novel--hatred.
Jack comes onto the island as the respected Head choir boy. Used to the leadership role, Jack questions Ralph's decision-making and true ability to lead the group of boys from the very beginning. In the novel's opening, Jack and Ralph bond as they explore the island, but Jack's feelings of goodwill fade as the other boys do not elect him as their chief, choosing instead to pick Ralph.
Jack feels the anguish and sting on his pride later in Chapter Eight, "Gift for the Darkness" when he attempts to rally the boys to choose him as leader:
"Hands up," said Jack strongly, "whoever wants Ralph not to be chief?"
The silence continued, breathless and heavy and full of shame. Slowly the red drained from Jack's cheeks, then came back with a painful rush. (127)
Jack's feelings of humiliation at not being chosen as the chief instead of Ralph quickly morph into anger and hatred. He storms away from the meeting, declaring, "I'm not going to play any longer. Not with you" (127). His savagery overrules his better emotions, and hatred takes over.
"The chief and Roger--"
"They hate you, Ralph. They're going to do you."
"They're going to hunt you tomorrow." (188)
Jack's resentment and humiliation at not being chosen as chief leads him to hate Ralph. Ralph stood up for what he believed, like when he told Jack that he could not steal Piggy's glasses, and reminded Jack too much of the rules of civilization. Jack answers this with defiance and hatred, both of which ultimately lead him to burn the island in an attempt to murder Ralph.
The main emotion Jack feels is the feeling of rejection. At the beginning of the story both he and Ralph seem to have a great deal of respect from the others boys. However, as the story progresses the boys seem to start to value Ralph's opinion and look to him more as the leader. This caused Jack's pride to be hurt and causes him to feel rejected. This sense of rejection causes Jack to react in extreme ways in order to try to maintain his sense of pride.