William Golding's novel Lord of the Flies is a symbolic novel which, as your questions suggests, traces the moral degradation of a group of English schoolboys. "Play" can be broadly applied to these boys, since everything they do might be considered play--for they certainly do not do much that could be called work. Several things they do which seem like play at first clearly foreshadow their moral degeneration.
First, they "play" with fire. When the suggestion is made to start a signal fire, they all make a mad rush to pile up the wood and only slow down long enough to figure out how to actually start the fire (Piggy's glasses). They squeal with delight and have a grand time, but the fire gets out of control and the result is a huge patch of ashy forest and one dead boy. This death is an accident.
Another form of play has to do with rocks. At the top of the mountain, Ralph, Jack, and Simon find a huge rock and lever it off its ledge, sending it flying down the mountain. Later, on the beach, one of the older boys throws rocks too near one of the youngest boys; he is kept from hurting him only by a sense that he should still be obeying the laws of civilization. These are moments of play which foreshadow the later use of rocks as weapons to kill. Eventually, Piggy is deliberately killed by a falling rock sent down on him as he approaches their fort. This death is intentional but could be justified as self-defense.
The most dangerous form of play for the boys is pig-hunting. Aside from Jack, who takes it seriously from the beginning, the rest of the boys just thinks it's fun to hunt. When they make their first kill, they are ecstatic. Later they kill a sow in a brutal manner, and soon they kill Simon as he steps into the hunting circle. Again, this might be considered self-defense. However, their final pig hunt, tracking down and killing Ralph, is unsuccessful only because the ship arrives. If it had not come, they would have killed Ralph deliberately, for no reason, after they hunted him down.
Each of these forms of play begins harmlessly but leads to something far more dangerous and deadly. Life is seen as less precious and their inner compulsion to violence grows stronger. These simple acts of play foreshadow the heinous acts of murder to come.