As the above post so well indicates, Simon, the peacemaker, and Jack, the warrior, share little but having come from the choister group.
Simon represents the intuitive nature of man, one which is acquired through much thought and mediation and communication with the inner civilized self. Much like Thoreau and Emerson, Simon seeks the peace and communication he has with Nature. And, much like Thoreau who went into the woods "deliberately to front the essentials facts of life, and see...what it had to teach," Simon goes into the woods where he senses that evil--the beast--is what lies inside each of them, waiting to be released through one's giving in to primal urges, urges which Jack surrenders his civilized nature to as he becomes a hunter/killer. When Simon is unable to communicate the truth that his inner self knows, Jack and the hunters kill him, to be certain that he will be silenced forever.
Simon, as we learn in chapter one, is one of the choisters of which Jack is the head boy. That is their common background. Jack is the smooth, charismatic leader who gathers his following by promising them instant gratification and by coercion. Jack is a bully who wants power. We see that trait in him with his first appearance when he forces the choir boys to march in the hot sun in their black robes. When he isn't elected leader by the boys, he decides to lure others away from Ralph with promises of food and fun. Eventually, when he has some strong henchmen (Roger and Maurice) on his side, he bullies some of the boys into joining his tribe. In the last chapter, we get the impression, he has resorted to torture to make boys follow his lead. He does not understand, nor does he want to understand, that the true beast on the island is the evil inside of each of them. When told that it was Simon that the boys killed in their frenzy around the fire, he angrily denies it saying that the beast may assume any form he chooses. Simon is the one in the group who is the first to fully understand what the beast is. Piggy knows that the beast the boys fear isn't tangible, but he doesn't understand what the true beast is. Simon is the peacemaker of the group with no designs on leadership. He'd rather go off by himself to think and to help others. He seems to innately know that their society will collapse and he dies trying to tell the boys the truths he knows.