How does William Golding suggest that the breakdown of civilized behaviour inevitable among the boys?
You probably do not have to look much further than the first three chapters to find your answer. First, remember how the boys came to be on the island. It was a result of a world war. If the known civilized world is not getting along, what hope is there for the youth?
You might also look at Ralph's early treatment of Piggy. His obvious disdain for him in Chapter 1 mirrors the boys' disdain for rational thought and intellect. Only a few of the boys learn to appreciate Piggy's gifts.
The next indication is the first fire. Ralph calls his first meeting as chief in which certain rules regarding parliamentary procedure are discussed, but the littluns are afraid of the dark and the unknown, and this fear continues to grow throughout the novel, creating disorder. When the boys decide a build a signal fire, their enthusiasm results in chaos. The boys' ignorance, lack of planning, lack of order result in the death of one boy and the destruction of a major portion of the island. We see early on how quickly things can fall apart.
Chapter 3 begins with a depiction of Jack--savage-like, almost madly obsessive--hunting. He is down on all fours, tracking the droppings, and disinterested in building the shelters. This image is another example of adopting the law of the jungle and succumbing to the savagery within.
We see very early two opposing methods of adapting: creating a civilization versus the survival of the fittest. The latter as represented by Jack seems strong and ominous, threatening the weakly established order represented by Ralph and Piggy.
I believe it is suggested when the boys first begin to divide up from one another. The reader identifies that the boys are two separate groups. One group serves the animalistic Jack and the other are the civilized followers of Ralph. However, Jack quickly woos the boys to his side not so much because evil is better but because the raw basic needs can be met through killing. Food is a basic need.
Civilization breaks down right away in the sense of the children abandoning the building of shelters and tending the fire. They play and dance, but they are not following the directives that could help them return to civilization.
By the time Jack has the dance around the campfire and Simon crawls out of the forest, the boys have responded like animals and slaughtered Simon.