How does Jack's character turn evil and why?Lord of the Flies by William Golding

The emergence of evil in the character of Jack Merridew is
central to Golding's theme in Lord of the Flies of the intrinsic
nature of evil in man.  Suggestions of the intrinsic evil in human
nature are made first through the character of Roger, who in Chapter Four sadistically
wishes to throw stones at little Henry who plays with the small crabs on the
shore:

Roger
stooped, picked up a stone, aimed, threw it at Henry--threw it to miss.  The stone, that
token of preposterous time, bounced five yeards to Henry's right and fell in the water. 
Roger gathered a handful of stones and threw them....Here, invisible yet strong, was the
taboo of the old life. Round the squatting child was the protection of parents and
school and policemen and the law.  Roger's arm was conditioned by a civilization that
knew nothing of him and was in
ruins.

Once the conditioning
of society is removed as it is on the island, the sadistic Roger and Jack, who is "a
continent of experience and feeling" apart from Ralph, begin their descent into savagery
as the evil nature intrinsic in them is free to emerge.  Having become the chief of the
hunters, Jack engages in primordial activities such as killing a pig and stealing the
fire. With such savage actions, along with his masking of his features, the primal urges
of violence and jealousy are given free rein in Jack and his inherent evil emerges as
his power over the others increases.