Because he is clever, thats why. Jack wants to hunt, to be active and lead boys into "battle". He knows that as long as the boys are scared, they are more likely to hunt. In discussing the beast with the group, Jack says this:
“You can feel as if you’re not hunting, but being hunted, as if something’s behind you all the time in the jungle.”
He is suggesting here that in order to survive, boys must hunt. Hunt is all he wants to do, to the extent of creating war paint to use in his hunting:
“He made one cheek and one eye-socket white, then he rubbed red over the other half of his face and slashed a black bar of charcoal across from right ear to left jaw.”
The hunting, just like his leadership of the choir boys in the first chapter, make Jack feel important and powerful. He hates that the boys have chosen Ralph as their leader and does all he can to undermine that, including flaming their fears about the beast, which will cause the boys to look towards him - the hunter - and away from Ralph.
It is Ralph, of course, that he ends up hunting in the end.