First, in theory, Hu Hai is not a suitable successor.
Hu Hai is not the heir designated by Qin Shihuang, and he is inferior to his brother Fu Su in both ability and character. At that time, Fu Su was the heir designated by Qin Shihuang, but Hu Hai became the heir to the throne after Qin Shihuang's death.
By rights, Hu Hai is not fit to be an emperor. But with the help of Zhao Gao, Zhao Gao was Qin Shihuang's confidant. Why did Zhao Gao support Hu Hai's accession to the throne? I think it's because of two points: "First, Zhao Gao was once the master of Hu Hai, and Zhao Gao definitely preferred Hu Hai. Second, Zhao Gao has a good relationship with Hu Hai, and Hu Hai can continue to be favored and take power after he succeeds. "
Second, Hu Hai's usurpation of the throne is recognized by history.
It is precisely because of Hu Hai's succession that Zhao Gaoshen protected his own interests to the maximum extent, and even thought that Hu Hai could gain more interests and power when he succeeded to Zhao Gao, so Zhao Gao would support Hu Hai's succession. But Hu Hai's succession led the Qin Dynasty to the abyss.
Originally, the heir to the throne should be Fu Su, the eldest son of Qin State, who is rich in talent and ability. However, due to Zhao Gao's intervention, the situation changed, and Hu Hai was promoted to many dissenting Confucian. As the eunuch and confidant of Qin Shihuang, Zhao Gao naturally saw Hu Hai and Zhao Gao being whitewashed in Zhao Zhengshu.
But it is an indisputable fact that Hu Hai usurped the throne. Assuming that he is really the heir to the throne, why kill so many brothers and sisters? This is completely inconsistent with the common sense of normal succession.