Simon is killed during a frenzied "dance" in which the boys, overcome by primal bloodlust and emotion, mistake him for the Beast. This marks a significant thematic turning point in the novel, because Simon had (arguably) represented spirituality and the possibility of the boys being called back from their degenerate state; his death is a symbolic blow to civilized power. Most incriminating is the fact that Ralph and Piggy participate in the murder, showing that even they are not exempt from these implications.
The next morning, Ralph comes to his senses and realizes what's happened, and attempts to reflect upon what they've done (this is at the beginning of Chapter 10). Piggy, however, is defensive and eager to make excuses. It's difficult to draw a single quote from this passage because it's largely conversational, but Piggy uses the reasoning that it was dark, there was thunder and lightning, they were scared, Simon was "asking for it" because he foolishly encountered the boys at the wrong time, and so on. Ralph counters that "it was murder" and that he wasn't scared. Piggy seems adamant that they have committed no crime, and even if they have, that they can be forgiven under the circumstances.
Jack makes very little mention of the issue. He and the other boys seem convinced that the Beast is real, and that whatever Simon was, he wasn't one of them, and that his death is not something to moralize over.