Jack killed the pig to show the others that he has no remorse for killing anything. This would create fear in the others and they might see him as being dangerous instead of fun.
Jack used Ralph's fear of the beast to gain support for his leadership and tribe. This happens because the tribe sees Jack as someone who can protect the tribe because of his skills in killing things.
Jack used fear to make his tribe obey him and to force Sam and Eric to join his tribe. He did this because of the competition between himself and Ralph.
The boys who are hunting climb through the burnt area of the mountain and reach the dead parachutist's body floating by the wind. This causes the boys to run off in terror.