What's Simon's theory of beast, whats he trying to communicate to the group and how well does he express what he thinks, and how do others...

While the boys discuss the possible existence of a beast, Simon summons his courage and speaks:

“What I mean is. . . maybe it’s only us...”

“...We could be sort of. . . ”

Simon became inarticulate in his effort to express mankind’s essential illness. Inspiration came to him.

“What’s the dirtiest thing there is?”

Mankind's essential illness ... the dirtiest thing there is. Jack says a dirty word and everyone cracks up. Simon's idea is lost amidst the release of tension and the laughter.

Simon sees what the others don't see, is beginning to know what the others will know too late. He will look at it face to face and it will talk to him from within. It will swarm with flies and make him ill... essentially ill.

We fear death and so we kill, as if in killing we can vanquish death itself. The dirtiest thing is the worst in us: the lust for power over fear and over death ... the need to kill for the sake of killing.

No one hears this, but they will.