The first reason is that Erhu and Rain is falling all have followed Zhu Yuanzhang and Ma Huanghou for too long, knowing too many secrets of Zhu Yuanzhang and Ma Huanghou. Although Xiaoxue has been taking care of Ma Huanghou, there is no problem, but erhu is Zhu Yuanzhang's confidant. Zhu Yuanzhang asked Erhu to do many things that he was inconvenient to do. Therefore, Erhu followed Zhu Yuanzhang for so many years and knew that Zhu Yuanzhang must have a lot of dirty things. Zhu Yuanzhang thought that if they were not killed, those dirty things might leak out.
The second reason is that Erhu offended too many people. Zhu Yuanzhang killed them, which not only saved them from being chased by the enemy, but also made them die more decently. Although many deaths and injuries were ordered by Zhu Yuanzhang, many times Erhu personally led a team to arrest or kill them. Those people dare not lay hands on Zhu Yuanzhang, but they know that Erhu and Rain is falling all will retaliate against them after they leave the palace. So Zhu Yuanzhang killed them, which can save some political influence and avoid them being insulted by the enemy.
The third reason is that Dahu died because Zhu Yuanzhang was afraid that Erhu would avenge Dahu one day. Although Hu Xiangjian proposed his death, Zhu Yuanzhang let him die in order to eliminate the influence. Although Erhu followed Zhu Yuanzhang for so many years, both Zhu Yuanzhang and Erhu knew how Hu Da died. Because Zhu Yuanzhang was afraid that Erhu would avenge Hu Da, he also wanted to kill him.