What an interesting task! I think the obvious topic(s) for your essay would be the murders of Simon and Piggy. These are the most blatant criminal acts in the novel. You could also investigate the attempted murder of Ralph when Jack and his tribe set the island on fire in an attempt to kill him or smoke him out of hiding. Their intent was murder even though they didn't succeed in their act.
For your thesis statement you need to make a claim about one or more of the murders. It would probably be a stronger paper to focus on ONE of the murders, just as an investigation would focus on one crime at a time. In the case of Simon your thesis could assert that Jack knew exactly who they were attacking, but that he did nothing to stop it because it gave him more power. On the other side, you could argue that the boys were all caught up in the mob mentality and the atmosphere of the reenactment so they are not guilty of first degree murder, but some lesser charge like manslaughter. Piggy's murder is a much more direct case of premeditated murder. Your thesis statement can make that assertion, and then your evidence would examples of the prior bullying of Piggy by everyone on Jack's tribe and Roger's earlier throwing of rocks and "just missing" as a build up to this murderous act.
No matter which topic you choose, your body paragraphs would need to explore the evidence of the boys thoughts, prior actions, motives and actions in direct relation to the crime.