Simon thinks the beast is not a creature but a human. why are humans scarier than creatures?

Simon is alluding to the evil in humankind, that when evil consumes someone they act in certain ways they wouldn't otherwise act. He concedes the evil exists and questions if it lives in the boys themselves.

"What I mean is.... maybe it's only us... We could be sort of.."?

Humans are scarier than creatures because humans possess morality (the ability to tell right from wrong) and still choose to do evil things.