For this theme, I've identified four quotes that all exhibit your identified theme. I'll summarize and analyze its presence in each afterward.
"'Aren't there any grownups at all?'
'I don't think so.'
The fair boy said this solemnly;but then the delight of a realized ambition overcame him. In the middle of the scar he stood on his head and grinned at the reversed fat boy.
Ralph, the fair boy, upon realizing that no grownups means a certain degree of liberty, becomes very excited. Without the rules of civilization to reign him in, he suddenly understands that he will have opportunities to do many things that society does not allow, a broad set of freedoms that include many actions. Some of these actions are brutal, violent, or dangerous. While those may not be the first things Ralph thinks of, they are included within the situation.
"Roger stooped, picked up a stone, aimed, and threw it at Henry — threw it to miss. The stone, that token of preposterous time, bounded five yards to Henry's right and fell in the water. Roger gathered a handful of stones and began to throw them. Yet there was a space round Henry, perhaps six yards in diameter, into which he dare not throw. Here, invisible yet strong, was the taboo of the old life. Round the squatting child was the protection of parents and school and policemen and the law."
Roger, like Ralph, is beginning to understand that a world without grownups is a world without certain rules and restrictions. However, Roger is unlike Ralph in two ways. Firstly, unlike Ralph, Roger does not wish to emulate the grownups by instilling order. He actually wishes to do things that are savage and destructive, such as throw rocks at Henry. Secondly, Roger is taking longer to realize that he does already have the freedom to defy grownups' rules, whereas Ralph has already had both this realization and an accompanying one: how dangerous people are with total freedom.
"[Jack's] mind was crowded with memories; memories of the knowledge that had come to them when they closed in on the struggling pig, knowledge that they had outwitted a living thing, imposed their will upon it, taken away its life like a long satisfying drink."
Jack thrives in a world without societal restrictions where his inner urge to savagely and violently destroy can be not only enacted but admired. Jack, after killing the first pig, reminisces about his natural savagery, indicating that it is something he enjoys and will continue to practice.
"There isn't anyone to help you. Only me. And I'm the Beast... Fancy thinking the Beast was something you could hunt and kill!... You knew, didn't you? I'm part of you? Close, close, close! I'm the reason why it's no go? Why things are the way they are?"
Simon hallucinates this conversation with the pig's head on the stake, or the Beast. The voice, which appears to come from the head on the stake but is of course coming from Simon's own head, clearly explains to the reader the phenomenon Golding hopes to explain with his novel: the Beast is a part of all the boys, whom without civilization have been freed to practice all of their inner destructive tendencies.