Why did Jack leave the assembly and run away into the forest alone?

Jack left the assembly and ran into the forest alone because he felt increasingly alienated from the other boys and rejected the constraints of civilization. His desire for freedom and power led him to embrace a more primal, instinctual way of life, distancing himself from the group dynamics and authority figures. This moment symbolizes his descent into savagery as he seeks to assert his independence and explore his darker impulses.