Ziying of Qin father Chengjiao
Being Qin Shi Huang's nephew
Ziying being Zhao Chengjiao's son bore no threat to Huhai's reign and was neither one of Qin Shi Huang's direct descendants nor in a higher position in the succession to Huhai. Ziying was also said to have tried to persuade Huhai not to kill Qin Shi Huang's other sons and daughters, which could have been a difficult task if he was among them.
This theory was more likely to be true than the other three.
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