Initially, the boys are relatively organized (thanks to Ralph, Piggy, and the conch) and peaceful. Over time, Jack's morbid interest in hunting increases. More and more boys are lured to his more barbaric camp and Ralph is left with fewer and fewer followers in his own camp.
The first instances of violence occur with the hunting party. In Chapter 4, Jack and his hunters return after having killed a pig. They are chanting "Kill the pig. Cut her throat. Spill her blood." Supposedly, they kill the pig for meat, for sustenance. However, their fascination with blood and the excitement of the kill suggests that the savage nature of the killing is more alluring than the goal of getting meat to eat.
There is a constant fear of "the beast" and this fear feeds into an attitude that encourages violence. At the end of Chapter 9, Simon goes to the beach to tell the others that there is no beast. (The beast is actually a product of their collective fears and potential towards violence.) But the boys on the beach are hyped up and in a frenzy. Chanting a similar mantra of killing, they kill Simon, thinking he is "the beast."
The beast was on its knees in the center, its arms folded over its face. It was crying out against the abominable noise something about a body on the hill. The beast struggled forward, broke the ring and fell over the steep edge of the rock to the sand by the water. At once the crowd surged after it, poured down the rock, leapt on to the beast, screamed, struck, bit, tore. There were no words, and no movements but the tearing of teeth and claws.
In Chapter 11, Piggy is killed and the conch is destroyed with him. This is symbolic on at least two levels. The most reasonable, intelligent boy is killed, trying to reestablish order. The conch, the symbol of order, is also destroyed. Violence has overtaken order. The smartest (Piggy) and the most peaceful (Simon) of the boys have been killed.