Why does Simon walk with "a sort of glum determination like an old man?

Simon is changed after his encounter with the Lord of the Flies. He has confirmed that any evil on the island lies within each boy. Still, Simon is Golding's Christ figure and he must try to save the boys from themselves,

The usual brightness was gone from his eyes and he walked with a sort of glum determination like an old man." Simon has lost his innocence; he knows more than anyone else about the beast, which is taking its toll on his body. ch9