I think what is meant by this quote is that, Simon's recongition of the Lord of the Flies is an evil that has always been inside of him. And he is seeing this evil in himself for the first time, this is ancient because Simon has always had this and like any person can not get rid of this. I think the Lord of the Flies is an inner evil that is showing itself to Simon, to test Simon in his capatilies like a "devil" of sense.-- Emily
I agree with Emily, here. It seems like Simon seems to be recognizing an evil within