If you are asking for examples of the children's lack of innocence, you do not have to look far. Roger throwing stones at Henry "just to miss" in Chapter 4 is an example of the cruelty that exists within the children. The only reason that he does not throw to hit is that he has been taught not to, not because he is innocent:
Round the squatting child [Henry] was the protection of parents and school and policemen and the law.
Jack's treatment of Piggy in Chapter 4--punching him in the stomach and face, breaking Piggy's glasses--is another. He hits Piggy because Ralph humiliated him by accusing him of neglecting the fire. He hits Piggy because he knows Piggy will not hit back. One chilling example of the boys' savagery is their killing of the the sow in Chapter 8. In this chapter they are portrayed as bloodthirsty individuals who kill for the sake of killing, not for food. Killing the sow makes the boys "fulfilled," and of course this bloodlust results in the death of Simon.
The reason for their lack of innocence might be found in Simon's conversation with the Lord of the Flies in Chapter 8. He tells Simon what Simon already knew that the beast is within. It is Golding's thesis that beneath the veneer of civilization, all human beings are savages. Once the rules and laws of civilization are removed, then our savage instincts are released, and there are no restraints.
One of my favorites is the contrast between the naval officer that comes to rescue the boys, his disappointment in how the boys appear to be fighting with each other and unorganized, while of course he is serving in a navy that is employed (at least until recently) in a war against another nation, involved in killing and destroying, just as the boys have been.
Another reason that Golding demonstrates for the boys being guilty of crimes is the obvious knowledge that Jack has of what he is going about doing as he incites the boys to be hunters and to kill and maim.
Perhaps the most obvious is the portrayal of Roger as being so incredibly malicious and clearly one of the few boys without any real compunction about killing or hurting other boys. Golding makes it clear that at some point, Roger begins to allow his malicious side to come out and it gets to the point that he feels no guilt at all.