It means they have blonde hair.
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It connotes danger as it shows that the island is hellish as well as heavenly
Piggy
It seems to mean that Jack's presence was drawing out the evil in Roger.
Simon is an epileptic and during an epileptic fugue he has an internalised conversation with the evil which he knows dwells within us all. He mentally projects this inner beast onto the pig's head on a stick which Jack has left as an offering for the imagined beast.