Prior to the death of Simon, there was a great deal of fear and guilt about the first death of a boy, even though they weren't sure when and how it happened, but likely in the fire that was started at the top of the mountain.
At Simon's death, the boys like Ralph and Piggy felt some responsibility and some shame and regret, as did the hunters. but Jack quickly erased this by creating the idea of a god-like figure that could disguise itself and so of course the boys lashed out and killed it.
So after Simon's death, and then during the confrontation at the rock fort, the boys in the hunting tribe finally lose (with some help from Roger) their fear of killing another human and they feel no shame after dropping the huge rock on Piggy. There is no longer any barrier or any difference between the hunt for beasts and the hunt for humans.
The murder of Piggy marks the boys complete abandonment of themselves to savagery. Jack has lost himself to his desire for authoritarian rule. Rather than depend on the ethical nature residing in each individual, the boys have blindly allowed Jack to lead them down his path. Their identities are also obscured behind their painted faces.
Roger feels absolute satisfaction from the entire situation. There is no sense of regret or evildoing in Roger. His cruel tendencies prevail with the death of Piggy and he has come into his own.