All of them die by violence in some way. The 'littlun' with the birthmark mysteriously disappears and is presumed dead; Simon is "sacrificed" in a barbaric way by Jack and his hunters; Piggy is smashed upon the rocks at the beach by a boulder pushed over the cliff.
Piggy, Simon and 'the littlun' also are set apart by some sort of physical 'mark.' For 'the littlun' the mark is evidently the birthmark, a rather insignificant imperfection. Piggy and Simon, on the other hand, have real health problems. Besides being clumsy and heavy, Piggy is nearsighted and asthmatic; Simon suffers from epileptic seizures and at times has hallucinations.
In the economy of his narration, Golding uses these physical weaknesses as an element of foreshadowing. On the boys' island and in their "fallen" world, there is no place for the weak or for the outsider, and nowhere safe to go. Most probably, Ralph would have also met a violent end had not the boys been found and rescued in time.