Jack's violent nature is foreshadowed in Chapter 2 through several incidents. At the very first meeting, the boys agree to vote on a chief as well as to establish rules. Jack announces:
"We'll have rules! Lots of rules! Then when anyone breaks 'em--"
Here we see Jack is more excited about punishing those who break the rules than the rules themselves.
When the boys attempt to build a fire, it is Jack who forcibly takes Piggy's glasses to use to start the fire.
"Later, when the question of a snake-thing or beast arises, Ralph quickly denies that there is a beast. Jack agrees with Ralph, but he adds,
"There isn't a snake-thing. But if there was a snake we'd hunt and kill it."
These three examples show a violent streak in Jack that intensifies throughout the novel.