Watching the savages retreat, Ralph tries to identify them as individuals and guesses one to be Bill. Then he realizes that, in fact, "this was not Bill" and he's right: Once divorced from his previously civilized self in appearance, behavior, and values, the individual who was Bill is gone. In the previous chapter, after Jack throws a spear with deadly intention at Ralph, Golding stops using Jack's name and refers to him as "the chief." The boy named Jack has been totally replaced with a primal entity, the personification of the beast's lust for power and the rejection of the civilizing forces represented by Ralph.
Even after the attack, Ralph so craves human companionship — the devil he knows — that he returns to Castle Rock to reason with Jack's tribe again on the next day, relying on their "daylight sanity." "Daylight sanity" is another term for common sense; Piggy tells Ralph in Chapter 8 that lack of common sense is the source of all the trouble on the island. At the time, Piggy referred to practicality, or a sound judgement of the actions they would need to take to attract a rescue ship and co-exist with some amount of civility.
