To identify quotes that illustrate that evil is inherent in everyone when it comes to William Golding’s novel Lord of the Flies, it might be helpful to define evil first, so that one knows what they’re looking for. Ideas about morals, justice, and evil have been on the minds of many famous thinkers, including Socrates, Friedrich Nietzsche, and Hannah Arendt.

Philosophy aside, evil is typically attributed to people and situations that feature a mix of inhumanity, apathy, and lethal destruction. Chattel slavery in the United States could be considered evil. Nazi Germany’s program to kill Jewish people could also be called evil. Of course, the island in Lord of the Flies can be labeled evil as well.

One evil quote to parse appears in chapter 9. At the feast, Ralph tries to confront Jack one last time. Ralph insists that he has the right to talk because he’s in possession of the conch. Jack replies, “the conch doesn’t count on this end of the island.” Jack’s repudiation of the conch conveys the idea that evil is inherent in everyone, especially if what’s stopping someone from evil is as delicate and dismissible as a shell.

Jack reinforces the idea that evil is inherent in everyone when he yells at his followers, “Do our dance! Come on! Dance!” This quote highlights the link between the boys’ capacity for evil and the way in which Jack draws it out. Without Jack’s encouragement, it’s possible to argue that the boys lack the impetus to kill Simon.

Another way in which Golding illustrates the theme that evil is inherent in everyone arrives via Ralph. After Ralph and Piggy witness Simon’s murder and return to their side of the island, Ralph is distressed. Piggy tries to console Ralph, but Ralph is adamant that they somehow participated in the evil killing of Simon. “Don’t you understand, Piggy?” says Ralph. “The things we did—.” Even though Ralph and Piggy didn’t do anything, their mere presence at the scene and the fact that they didn’t intervene appears to make them evil. They revealed their capacity for evil because they didn’t try to stop Jack’s faction from committing evil.

I would highlight the chant that the choir boys yell while on the hunt:

"'Kill the pig! Cut his throat! Kill the pig! Bash him in!'"

Combined with Ralph's observations when finally faced with the pig itself:

"The desire to squeeze and hurt was over-mastering."

And, finally, any description associated with Roger, the most evil of the boys:

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's arm was conditioned by a civilization that knew nothing of him and was in ruins.

Some of the best quotes that illustrate evil is inherent in everyone come from Simon. In Chapter 5, when the boys are arguing about the beast, Simon says, "Maybe the beast is us", indicating that Simon under the truth that the beast (evil) is inside each one of us. During Simon's hallucination, the beast says, "I am a part of you," Golding seems to imply that no matter what name you give to evil, be it sin, the devil,neurosis, hate, violence, terrorism, or sheer wanton destruction, these traits are inside of man. This idea is confirmed when Simon discovers the beast truly is a man in the form of a downed parachutist. When he tries to bring the truth to the rest of the boys, they kill Simon in a frenzy--pointing to Simon and saying "Kill the beast"!

In order to select quotes which illustrate the theme 'evil is inherent in everyone' it is important to refer to the context of the quotation as much as the words themselves. The final chapter, where 'Ralph wept for the end of innocence' should be reflected on to consider whether the boys ever were innocent, or just ignorant at the start. Ralph, by the end of the novel, has begun to understand the evil found 'in the darkness of man's heart'. Of course the quotation refers to mankind, but it should be noted that technically none of the major characters are men. We have seen evil committed in the story--cruelty, betrayal, murder--and yet what could be most chilling is that these are the crimes of children: the last vestige of goodness. This would clearly illustrate that 'evil is inherent in everyone'.

I agree with the above poster that the dialogue between Simon and Lord of the Flies is a good start. You might also have a look at the moments of the two deaths - Simon's and Piggy's: particularly the way that even the good characters like Ralph are drawn into complicity with the murderous tribal behaviour. For Ralph, remember, in the hunts, "the desire to squeeze and hurt was overmastering".

You might also have a look at the moments of the hunts and compare the communal mentality of sadism (as embodied best by Roger: "Right up her a**e") here to that at the two murders.

And don't forget that hugely important last page - look at why Ralph cries: "the darkness of man's heart".

The dialogue between Simon and the Lord of the Flies would be a good place to start.  The Lord of the Flies taunts Simon with the idea that there is no escaping him, and, if we see him as the symbol for mankind's inherent evil, then this means that Simon learns he cannot escape his own inner evil.  He then leaves to warn the others that this is what they must all fear, not some imagined beast.