Ralph tries to be a leader in order to get things done, and Jack wants to be a leader to get personal power.
Ralph is chosen leader almost immediately, because he makes the first move of leadership on the island when he blows the conch. The conch seems to imbue him with special powers. The boys are confident in him because he has the conch, even though Jack is the more obvious leader and Piggy is the only one who shows intelligence.
Ralph is not a capable leader, however. He simply can’t get the boys to listen to him. They only get two shelters built, and they keep letting the fire out. Eventually, the boys break into two factions, and Jack takes most of the older ones.
Jack seems more interested in violence and power from the start. When the boys are voting, he is most interested in having rules because of what he can do to those who break them. We later see that he does not need someone to break the rules to get physical.
“…He got angry and made us tie Wilfred up.
He’s been”—he giggled excitedly—“he’s been tied for hours, waiting—”
“But didn’t the chief say why?”
“I never heard him.” (ch 10)
Jack is a heathen. His hunters are bloodthirsty, and move from killing pigs to killing kids. They lose themselves in the anonymity of anarchy, and soon enough there is little civilization left.