Since enotes editors are only allowed to answer one question at a time, I had to edit your question. The "taboo of the old life" that you refer to in Chapter 4 occurs in the following passage:
Roger stooped, picked up a stone, aimed, and threw it at Henry--threw to miss . . .Yet there was a spce round Henry, perhaps six yards in diameter, into which he dare not throw. Here, invisible yet storng, was the taboo of the old life.
As the smaller and younger boys are playing in the sand, Roger and Maurice are lurking behind them. Roger begins to throw stones at Henry. But he does not throw to hit. Rather, he throws to miss. Early in the novel, Henry is protected by the rules of their old life--the rules that the boys have been taught since they were infants: do not throw rocks at our friends; do not hurt others. There are certain taboos (rules prohiting certain behaviors) that we all learn very early in life, and it is these rules that keep Roger from doing real harm to Henry.
This section is particularly frightening because the reader is well aware of the fact that while Henry is protected by the teachings of his "parents and school and policeman and the law" that none of these authority figures exist on the island. We have to wonder what will happen when it occurs to Roger that if he threw stones to hit there would be no reprecussions for his actions.