Simon is a Christ Figure for many reasons:
Compassion for others - Just as Christ showed compassion for others in his healing of the sick and feeding of the hungry, Simon shows compassion in the way he stands up for Piggy when Jack criticizes him (ch. 2), helps Ralph with building the huts after others have abandoned the work (ch. 3), and helps the littluns get food that is out of their reach. This last act is seen in chapter 3:
Then, amid the roar of bees in the afternoon sunlight, Simon found for them the fruit they could not reach, pulled off the choicest from up in the foliage, passed them back down to the endless, outstretched hands.
Quiet time alone - Just as Christ often leaves his disciples for short periods of time so he can be alone to pray, Simon leaves the others so he can go to his "quiet place" for time alone. Golding describes Simon's "oasis" at the end of chapter 3:
When he was secure in the middle he was in a little cabin screened off from the open space by a few leaves. He squatted down, parted the leaves and looked out into the clearing. Nothing moved but a pair of gaudy butterflies that danced round each other in the hot air.
Spiritual Insight - Just as Christ showed great spiritual insight and shared these insights through his teachings, Simon also has a strong spiritual sense of what is true and right. He shows this in chapter 3 after Ralph raises the concern to Jack that the littluns are afraid:
(Ralph): “They talk and scream. The littluns. Even some of the others. As if—”
(Simon): “As if it wasn’t a good island.”
Astonished at the interruption, (Jack and Ralph) looked up at Simon’s serious face.
“As if,” said Simon, “the beastie, the beastie or the snake-thing, was real. Remember?”
Additionally it is Simon who later states, when the older boys debate whether there is a beast, "Maybe it's only us" (ch. 5). When the other boys ridicule him, Simon becomes "inarticulate in his effort to express mankind’s essential illness," finally saying:
“What’s the dirtiest thing there is?”
As an answer Jack dropped into the uncomprehending silence that followed it the one crude expressive syllable....
Simon’s effort fell about him in ruins; the laughter beat him cruelly ...
Criticized and mistreated by others - The above quote also shows how Simon is ridiculed - something Christ experienced many times throughout his life. Even Piggy calls Simon "cracked" (ch. 8).
Temptation in the wilderness - Just as Christ is tempted by Satan when he goes into the desert for 40 days, Simon is tempted by the "Lord of the Flies" (aka. Beelzebub, or Satan) in his conversation with the pig's head in ch. 8. The head urges him:
“Well then ... you’d better run off and play with the others. They think you’re batty. You don’t want Ralph to think you’re batty, do you?... Aren’t you afraid of me?... There isn’t anyone to help you. Only me. And I’m the Beast.”
Mission to bring the truth - After this confrontation, Simon sets out to learn the truth. He climbs the mountain, discovers the "beast" is only a dead parachutist, sets him free, then goes to carry this message to the others, just as Christ sought to bring his message of truth to his followers.
Persecuted unto death - Just as Christ was killed on the cross after the angry mob shouted for the Romans to "crucify him!", Simon is killed on the beach by the other boys. Unlike Christ, however, there is no resurrection for Simon.