In Chapter 4, Roger and Maurice begin to bully the littluns who are playing on the beach. After Roger and Maurice destroy the littluns' sandcastles by running through them, Roger follows Henry along the beach from a distance. As Henry plays with the tiny transparent creatures washing ashore, Roger begins to throw stones in Henry's direction but purposely aims to miss. Golding mentions that there was an invisible barrier around Henry that Roger refused to throw inside. In their old lives back at home, it was considered taboo to throw stones at a person. Roger's self-control and refusal to throw the stones directly at Henry reflects how he was conditioned by civilization not to hit others with stones. Roger's actions reflect this taboo because he purposely aims to miss. As the novel progresses, Roger forgets his conditioned civilized responses and becomes the most ruthless savage on the island.
The "taboo of the old life" that Golding refers to here is the taboo against hurting someone unnecessarily; against being savage. Roger is throwing stones at another of the children, Henry. Roger intentionally throws the stone to miss Henry. He throws it to land a few feet from him. His arm, according to the story, is conditioned to avoid hitting the boy because of that taboo against harming another person. The stone itself, a relic formed in ancient times, is symbolic of those ancient times when savagery was the norm because savagery often meant survival. It is only the remembrance of civilization that keeps Roger at this point from hitting Henry. Sadly, that civility will leave Roger soon.
The word "Taboo" can be seen as prohibition from religious or social conventions, making this part of the novel all the more evidence for Golding's book as a microcosm of World War 2. Here taboo could be seen as a comparison to the military background and is synonymous of the way many people are fighting in something that they don't undestand. Similarly Maurice and Roger are still attached to the "taboo of old life" that they were forced to live by previously and have kept these rules because they are bound by the constraints of society despite not understanding their purpose.