Add the settings for Simon's peaceful place on the island
The beast is the evil that each boy is capable of. It that running back to the other boys won't do him any good. The beast is a reminder that there is evil in each one of them, even Simon.
Add how Jack destroys Simon's peaceful place
Add the meaning behind the title. In Biblical sources, Beelzebub is another name for the devil which directly translates to "Lord of the Flies"
Simon falling into the mouth is that evil is going to consume Simon. It foreshadows Simon's death by the violence that will soon be unleashed from inside the boys.
Add the meaning behind what is symbolizes when the pig's head says "I'm the beast"
Add an artistic representation of what the Lord of the Flies sees inside Simon....inside everyone
Add the deeper meaning behind what Simon sees inside the mouth of the Lord Of the Flies