Tensions reach a high point in chapter 7 when it becomes clear that Jack is challenging Ralph's leadership. Jack is starting to do whatever he wants.
From the beginning, Jack has resented that Ralph was elected leader. Ralph is naturally good-looking, athletic, sensible, caring, and charismatic, making him the natural choice, but Jack has a will to power, and as head of the choir boys, he wants to be put in charge. A clash has been inevitable and now is coming to a head.
Ralph realizes for the first time how much Jack hates him. Jack asserts his own authority by saying that he is going to go up the mountain to look for the beast:
“I’m going up the mountain.” The words came from Jack viciously, as though they were a curse.
Ralph realizes that it is foolish to make such a trek, but when Jack sneers at him with the word "Coming?" Ralph knows he has to go along to maintain power.
Ralph is highly frustrated with Jack's behavior. Ralph, a good leader, behaves rationally and looks out for the interests of all the boys. He is willing and able to defer present pleasure for later rewards. Jack, on the other hand, has grown impatient with this form of leadership. He wants power at all costs and doesn't care who gets hurt or what gets destroyed as long as he gets it.