He was only frustrated because of Ralph's leadership
He knew he could lead the group, but his main downfall was attempting to dominate the group rather than working with Ralph to lead. He felt his hunting skills made him a better leader compared to Ralph, and maybe if he was appointed chief from the beginning, the following events could have panned out differently.
Jack was leader of the choirboys in school, arriving to the island with experience leading a group. He felt compared to Ralph, he was the obvious choice. When Ralph was chosen, he didn't understand why the group preferred Ralph over him. Ralph had structure and rules, and meant well for the group and their survival, but Jack shown his own abilities in terms of hunting pigs and leading the hunting group of the boys. He felt as though hunting was more important than the rules and ideas Ralph created. The sightings of the beast fueled Jack to finally challenge Ralph, and show the boys who their chief should have initially been. He used the beast to convince the others to join him as they would be protected from the beast being under his leadership. Not wanting to listen to Ralph's rules and ideas any longer, he goes rogue and convinces some of the group to follow his leadership. They could have simply co-existed on the island trying to survive, but the feud between Ralph and Jack over chief led to nothing but chaos. It is not told why so many boys slipped off to join Jack's group when they refused to vote for him as chief, but his plan to face the beast and protect them may have caused a change of heart. Though Ralph was kind and meant well, the boys may have felt Ralph would not be able to protect them in terms of hunting and facing the beast.
Ralph is patient and exercises good judgement, while Jack is ruthless and abandons any structure or routine. The groups' choice to appoint Ralph as chief in the beginning of the novel was the right choice.
[P1] Jack felt as if Ralph was too weak to lead the group. [P2] He was not evil, simply frustrated.
Rejecting the premises
[Rejecting P1] Ralph may have lacked hunting skills, but he wasn't too weak to lead.