As the boys first try to start hunting, it is really like a game, something they'd imagined in the past but they have no real idea of how to do it, and many of them lack the stomach to do the actual killing when the time comes.  As time goes on, they grow hungrier and more desperate and their games also become more violent.  It goes from them horsing around and pretending to kill one of the boys but they pull back when it is apparent that they have actually pushed too far and hurt him.

Then, in chapter 9, their violence reaches an entirely new level as they are playing their game and dancing around the pig but out of fear and this dark animal desire to hurt, when Simon emerges from the woods they mistake him for the beast and brutally murder him.  This escalation in their violence serves to highlight the theme of the breakdown of civilized instincts in the boys as their "play" leads to murder.