Jack has just discovered "the hunt" and is about obsessed with it. So if there is a beast (which he doesn't really believe at this time), he will hunt it. Piggy knows scientifically that it's not possible. Simon knows that there is no beast, and Ralph even says that it's not possible. But Jack is the aggressive one and wants to HUNT it.
"Ralph's right of course. There isn't a snake-thing. But if there was a snake we'd hunt it and kill it. We're going to hunt pigs to get meat for everybody. And we'll look for the snake too--"
he basically just said he would kill it.