Simon is the only boy that has an understanding of what the real "beast" is. And the lord of the flies is acknowledging his understanding and pushing him a little bit farther. Because the lord of the flies is representing the evil that exists inside all of the boys, it uses the plural pronoun "we."
After Simon receives this revelation, he goes to the mountaintop to figure out what the other boys saw and then down to the beach to share the revelation. One example of the "fun" that they are going to have comes when he emerges from the jungle in the midst of the boys party. They immediately react in terror and brutally kill him.
Another example of the "fun" that the beast has on the island comes after Roger has found that the rules of the old world no longer apply to him. He decides to kill Piggy with the boulder he can push off the cliff.