The "beast," is an embodiment of the boys' growing levels of insanity and parania. There was no real beast; one was the corpse of a soldier, the other a pig head on a stick. When Simon comes to understand that the beast on top of the mountain is only a deceased pilot, he foes to tell the others. The insanity and blood lust that ensues is what kills Simon. The beast reveals human mature in it's wild state and about the primitiveness that dwells deep within us all. (I hope you don't mind my round-about way of answering your question, I felt compelled to go much more in depth than what was expected.)
To quote directly from what Jack said in chapter 10, The Shell and the Glasses... "He came--- disguised. He may come again even though we gave him the head of our kill to eat. So watch; and be careful."According to Jack the beast is able to deceive the boys because he was wearing a disguise. He tells the boys the beast will be harder to recognize because he will continue to come in different disguises.
In reality, the beast is really the savagery inside the boys, so it changes as they change. However, Jack uses the argument of the beast's deception to keep them afraid and under his control.
Jack's answer to questions about the beast is to say that, "if there is a beast we will hunt it and kill it." Ralph keeps insisting, "but there is no beast!"
they went after the beast but jack decide to forget the beast and play rolling rock
Kill it.
by killing people and giving them to the beast "sacrafice"
Jack says they will leave the head of a sow for the beast as an offering in Chapter 8. He believes it will appease the beast and prevent it from attacking the boys.
Jack's denial that they killed the beast reveal about him that beast still exists. His plan that they keep on the "right side of the beast" suggest that people frequent make offerings to satisfy the beast. I think Jack truly believe there's a beast because he had seen it on the mountain top. His continuing to promote the existence of the beast to the others give him advantage over them by making obey him if they want to be safe!
jack
he offers his tribe his protection from the beast om the jungle
Jack needs the power of the beast in a couple of ways: as a common enemy for the boys to unite them; as a way to gain respect and to manipulate others as well as to satisfy his hunger for power; and as an excuse for hunting perhaps. Jack sort of became the spokesperson in the previous few chapters. He give offerings to it, tell others to leave it alone on 'the other side'. He is trying to make it more real for the others. Jack himself does not believe in the physical beast but is closer to the true beast- the capacity for evil in everyone, then everyone, even Simon. He is not afraid of it, as no one would use things they are uncertain or afraid of. if the beast is dead, he will loose power. Therefore even though he consciously recognize the murder of Simon though he hides it from the others. No one could stop him from using the beast now that Simon has died. He could make the beast immortal.
Jack blows the conch for the assembly to talk about the beast and that Ralph thinks that Jack's hunters are cowards and can't face the beast.