Environment plays a key role in shaping a person's actions. When the boys become stranded on the island, their perceptions about what behavior is acceptable and normal go out the window. Environment absolutely dictates actions; people act differently at church than they would, for example, at a football game.
The boys on the island allow themselves and their beliefs to be influenced by the environment.
Here is how I would approach this question through subtopics. I would address different aspects of the environment of the island.
Harshness of the untamed island--Forces the boys to go into survival mode. In the beginning, the boys cannot afford to consider the niceties of behavior, because survival is key.
Lack of structure/civilization-- Without parents or teachers to guide them, the boys must decide what is right or wrong or how to act. They attempt to form a sort of structure with the conch, but do not possess the self-discipline to enforce it.
Isolation Factor-- As the boys are stranded and lose hope of rescue, many of them use the isolation of the island as an excuse for their behavior and actions, like 'it doesn't matter what I do now, because we're never getting rescued, and my parents will never find out.' People will act certain ways behind closed doors that they would never act in public, and in many ways, the island is like being behind closed doors for the boys.