That question is so hard. Only two events? So much happens in the book.
Number 1. The election of Ralph as chief instead of Jack. Jack obviously wants the position, and he is not thrilled about losing to Ralph.
“I ought to be chief,” said Jack with simple arrogance, “because I’m chapter chorister and head boy. I can sing C sharp.”
The reason the event is so important is because it immediately pits Jack against Ralph. They may get along at first, but Jack is clearly biding his time to seize power. If Jack had been voted chief right from the start, the group of boys might not have torn itself apart over which boy to follow.
Event #2. Simon's death. Simon is the only boy that is inherently good. He is pure altruism and believes in doing right for the sake of doing right. When he is killed, it signals the complete loss of innocence and good among the boys. After Simon's death, mutual respect and order fall apart. Jack seizes full control, rules with an iron fist, and severely punishes anybody that doesn't explicitly follow his orders. Simon's death is the equivalent of a person losing their conscience. Only this time, the loss affects an entire group.