Secrets of the Pre-Qin Dynasty

Qin, one of the vassal states of the Spring and Autumn Period and the Warring States Period, was not the strongest but had the longest history, but it finally unified the land of China as if blessed by God. However, after the First Emperor ascended the throne, he died in just two generations. Why is this? Since Shang Yang's reform, Qin has officially risen, and all the answers seem to come from this.