How does the third death on the island differ from the other two deaths?
The deaths in the novel can be categorized in much the same way that our legal systems distinguishes them: first degree, second degree, and third degree murder.
The first death occurs when the boy with the mulberry birthmark succumbs to the fiery flames of the very first signal fire. Here the death could be considered third degree murder--murder, because it is the boys' carelessness and thoughtlessness in building and tending to the fire that results in the death of this littlun. It was not an intentional death, but the boys are somewhat culpable.
The second death--Simon's--is more like second degree murder--a murder which occurs in special circumstances, usually involving passion or rage. In this case the boys' fear of the beastie and their need to release the growing tension among them result in the death of Simon. There was intent to kill, but the murder was not premeditated. All the boys are culpable in this death--either by participation in the killing or by their passivity, doing nothing to prevent Simon's death.
The third murder is first degree murder. Piggy's murder is premediated. Roger on Jack's command uses a lever to topple the big rock onto Piggy's head. There is no remorse for this action, and no attempt to excuse this action, as many of the boys did in the aftermath of Simon's death.
Each death represents the esculating savagery on the island as the deaths progress from accidental to cold-calculated murder.