In Chapter Five of Lord of the Flies Piggy explains to Ralph that Jack could be very dangerous if not to Ralph then to whom?♥
By the end of the failed, twilight meeting Ralph is ready to quit as leader. Jack has shown complete contempt for Ralph's leadership and Ralph is in despair. However, Piggy and Simon urge Ralph to continue. For Piggy's part it seem to be about self-preservation,
"If you give up," said Piggy in an appalled whisper, "what'ud happen to me?" (p.116)
Piggy goes on to explain his belief that Jack simply hates him and wants to hurt him. He is at a loss to explain exactly why, but he tells Ralph that the hatred extends to him as well. We again see Piggy as the wise figure of the group as he is able to grasp the reasons why Jack hates Ralph (Jack isn't the leader and Ralph humiliated him over the fire). Yet according to Piggy, Ralph won't be harmed by Jack as he (Ralph) would be able to physically defend himself. This isn't the case for the physically vulnerable Piggy, and he knows that without Ralph's protective leadership he may well be left open to Jack's hateful attacks,
"...I know about people. I know about me. And him. He can't hurt you: but if you stand out of the way he'd hurt the next thing. And that's me" (p.116)
We know Piggy's words to be accurate, but perhaps as the reader we can guess that Jack's hatred of Piggy simply comes from that awful, dark part of human nature where people (especially children) simply hate the weak and vulnerable because they can. We also later learn that Piggy has underestimated Jack's hatred of Ralph; it will boil over to such an extent that he will in fact physically challenge his rival.