Jack, at the beginning of the story, is like a seed. By the end of the story that seed has sprouted into something wicked, but in the beginning is only that seed.
First, he is assertive. Take a look at this quote:
Merridew turned to Ralph.
“Aren’t there any grownups?”
Merridew sat down on a trunk and looked round the circle.
“Then we’ll have to look after ourselves.”
Notice how quickly that Jack decides that they will need to look after themselves. He accepts it as a fact and shows no emotion.
“You’re talking too much,” said Jack Merridew. “Shut up, Fatty.”
Jack is cruel. He has no problem "putting Piggy in his place." He feels no remorse for this...it is simply the way things go.
Later on, a clue is given in the fact that Jack is not able to kill the pig. He just doesn't have it in him. It isn't until he paints his face in the "mask" that he is transformed into the full-fledged Jack we know and love.