Does Jack's desire to kill a pig fit into either of the group's two main priorities established by Ralph?
Ralph mentions the boys' main priorities in chapter two, after he, Jack and Simon had established that they were on an island.
“Now we come to the most important thing. I’ve been thinking. I was thinking while we were climbing the mountain.” He flashed a conspiratorial grin at the other two. “And on the beach just now. This is what I thought. We want to have fun. And we want to be rescued.”
Ralph's assertion that they want to have fun can be understood since he is a boy. He realizes that there will be no adult supervision and it is in their nature that boys will want to enjoy the freedom that this brings to do whatever they want, whenever they want.
In this context then, yes, Jack's desire to kill a pig fits in. Killing pigs will be a fun activity for him. There is no adult around to stop him from doing. It is clear from their earlier experience with a piglet that it is Jack's wish to kill. He could not bring himself to do it during this first encounter for the enormity of the deed overwhelmed him and stayed his hand. He did clearly state, though, “I was just waiting for a moment to decide where to stab him” and later mentioned that the animal broke loose before he could stab it.
Jack's desire to kill, though, does become problematic in terms of the second priority, the need to be rescued. The boys decide that a signal fire should be made to alert any passing ships to their presence. Once the fire gets going it has to create a smoke signal and, for it to be effective, it needs to be maintained. Jack volunteers that he will divide his hunters into groups to take care of the fire and keep a lookout.
Later, in chapter four, though, when Jack and the hunters go off, they become so involved in the fun they have that they forget about the fire. Jack mentions that he needed the twins, who were tending the fire, for they needed them to form a ring to capture and kill a pig, which they have successfully done. The fire fizzles out during a crucial time - when a ship passes by on the horizon. Ralph is angry about the missed opportunity for rescue and vents his frustration by scolding Jack for his and his group's irresponsible neglect.
“There was a ship. Out there. You said you’d keep the fire going and you let it out!” He took a step toward Jack, who turned and faced him. “They might have seen us. We might have gone home—”
This confrontation creates greater division between the two boys and introduces a dramatic turning point which would affect every event that follows.
At first it would seem that this desire to kill a pig would fit into a priority for survival for both of the groups. However, Ralph clearly points out several needs. He notes the need for shelter and the need for rescue. These can be attended to by building shelters and building and maintaining a fire so that a smoke signal can be seen by passing ships or aircraft. Ralph also notes that fresh water must be maintained for the boys. A process of going into the island and getting steam water and filling coconuts with it has begun. Unfortunately, some people are not following through. Another goal is to maintain appropriate bathroom facilities so that feces are washed away by the tide.
Jack's priority to kill a pig does not coincide with Ralphs goals of safety, shelter, cleanliness, and sustinence. It will compliment the goal of sustinence, but I have a feeling Jack wants a pig for the wrong reasons.