"He says he saw the beastie, the snake thing, and will it come back tonight...he says in the morning it turned them into things like ropes in the trees and hung in the branches."
"The mask was a thing of its own, behind which Jack had liberated from shame and self-consciousness"
The beast was the first decent to savagery. Once they had in their minds that there was a beast they started acting like beasts.
When Jack painted his face he let go of his civility in that moment. The face paint made him feel like he could do anything no matter how savage it was.
By him stood Piggy still holding out the talisman, the fragile, shining beauty of the shell. The storm of sound beat at them, an incantation of hatred. High overhead, Roger, with a sense of delirious abandonment, leaned all his weight on the lever.
Another big descent to savagery was when Jack and his choir killed the Mother pig. They killed it in a gruesome way and it was not just to get food, it was the need to kill. Putting the pigs head on a stick for the beast was them showing how they were the beasts.
Killing Simon was one of the biggest events in this book. They definitely let go of their civility in this moment. They thought they saw the beast so their savage response was to kill it without even making sure it was the beast. The fact that none of them took responsibility for killing Simon also showed their savagery.
When Jacks choir went and stole piggy's glasses and fought with Ralph and his group, showed the boys savagery. They had fallen so far from civility that they didn't care what they did to each other. They only cared about themselves.
When Jack and his choir chased down Ralph to kill his was the final descent to savagery. They were so savage that they were actually trying to hunt down a person and kill him. At this point they had no civility whatsoever.