When the boys first come together on the island, there are all kinds of signs of civilization. The choir boys led in a procession, wearing uniforms, the idea of a conch being a symbol of cooperation and attention so that the boys can use it and if they have it, it is their turn to speak, all of these are signs of civilization and order.
There are small signs of a breakdown at first, boys wanting to hunt or whining and crying, then the little kids going to the bathroom where it is convenient instead of in the designated area, the worries about a beast, etc.
These start to build as there is a clear conflict between the leadership style and objectives of Ralph and Jack and this drives the story until the two groups split apart.
There are several instances that demonstrate the final breakdown, the killing and maiming of the pig, then the killing of Simon, then probably the last straw is the killing of Piggy as he stood for the aspects of civilization like science and reason.