There are several ways in which Golding suggests that the civilized behavior of the boys will inevitably come to and end. Let me give you a few of the instance where he suggests this point.
First, right from the beginning when there are two groups of boys, we can sense tension. Ralph is one of the leaders and Jack is the leader of the choir boys. There is obvious tension between the two.
Second, as the story progresses, there are two clear sides. There are those who want to hunt the pig and are obsessed with this and the others. In fact, this division is more serious, because there is a learned allusion to Plato's Republic. The boys want meat and according to Plato it is the desire for meat that leads to many vices.
Finally, the breaking of the conch is the final allusion that all is lost. As you recall, it is the conch that brought the boys together. It was a symbol of civilization.