How does Simon interpret the beast?

The beast is manifested as an external force in the island. Of course it is Simon that the only threat on the island is themselves, "Maybe there is a beast....maybe it's only us". As the boys hunt for an external beast Simon has his own vision:

“However Simon thought of the beast, there rose before his inward sight the picture of a human at once heroic and sick.”

Simon knew that the only beast lay inside the human soul. Simon interprets humanity with the capacity for good and evil. Humans are capable of kindness but possess a sickness that draws them to darkness. It is a paradox that Simon struggles with throughout the novel.