1) The boys are having a jolly game of pretend--we're all on this island and we're going to have fun. Let's name a chief--yeaeaeaeah! Suddenly fear is introduced, and it's not a game anymore. Now they truly need a leader. Rescue is more important. Later, Jack capitalizes on this fear.
2) Jack is absolutely a better leader. He is able to make the boys do what he says. He does this through fear. Unlike Ralph, who simply asks the boys to do what is right, Jack demands obedience and punishes disobedience. In fact, he beats Wilfred for no apparent reason. (As George Orwell puts it in 1984, power is the ability to make another person suffer.) So, all the boys do what Jack says. When boys don't do what Ralph says, he is simply stumped and cannot understand why they won't do what is right. Jack has a better understanding of people's motivations. We are all naturally lazy and selfish and violent--that's the Beast in us. While Ralph insists that there isn't a Beast, Jack says we'll hunt it and kill it. He knows such a thing is not possible, but he uses that fear and apparent power to take control of the boys. Ralph is the better man, but Jack is the more effective leader.