What is evil about Jack's pig-hunt?
As the previous post points out, the hunts grow to be more and more evil as time goes on. But one of the important things that emerges, in terms of evil, is the real character of Roger and this is only enabled by Jack and his pushing the boys towards hunting and giving Roger the chance to show his true colors.
In many ways, it is Roger that stands for the evil that Jack brings about with his willingness to dissolve the organization that Ralph had tried to hold together. Jack is still aware of his fear of certain things and that certain things unnerve him, but Roger is full of nothing but malice and evil and eventually turns his violent eye on the boys after focusing some of it during the hunts previously.
Jack and the others go pig hunting a lot, so I do not know if you are referring to one particular hunt or not.
In general, the hunts are not exactly evil, especially at first. The hunters are more irresponsible and less evil at first.
But then things get worse. For one thing, you have the boys start to do more and more violent things after they get back from hunting. Second, when the boys manage to kill a pig for the second time, they brutalize the pig in ways that are really unnecessary. At that point, they are no longer hunting for food or even for adventure. Instead, they are trying to hurt things and you could say that is evil.