Why do you think Jack insists on going up the mountain to look for the beast even though it's already dark when they arrive?

Jack insists on going up the mountain to look for the beast because he wants to assert his leadership and show that he is brave and in control. Additionally, he may believe that by facing the beast head-on, he can prove his strength and intimidate the others into following him. Finally, Jack's desire to confront the beast may stem from his increasing obsession with power and control on the island.