Let us remind ourselves what leads up to this statement. The boys have just played a rather disturbing "game" where Robert pretends to be a boar and the boys pretend to hunt him. Of course, the game turns into something else completely as the boys seem to be overpowered by the desire to wound and kill. Even Ralph, the character who tries to maintain civilisation amongst the boys, shows how this blood-rage overpowers him. Note how he responds as the game develops:
Ralph, carried away by a sudden thick excitement, grabbed Eric's spear and jabbed at Robert with it.
Even when the game turns nasty, with Jack trying to cut Roger's throat and thereby "kill the pig," Ralph fights to get near, wanting a "handful of the brown, vulnerable flesh." This "desire" is "overmastering" to him. According to the boys, this was a "good game," and as they talk about actually using pigs, Jack's suggestion to use a littlun instead eerily foreshadows the violence that the boys will enact on each other and their descent into savagery.