In fact, Zhu Yuanzhang did not let Erhu and Yu'er go, the young couple was finally killed by Wu Feng's men, but just did not take this shot at that time. On the surface Zhu Yuanzhang let Erhu and Yu'er go with their children, but behind the scenes Zhu Yuanzhang let Wu Feng follow them with his men. And from that time Wu Feng said to the people around him: you two, go down. This also meant that those two people were told to kill Erhu and Yu'er, so in the end Zhu Yuanzhang didn't let these two people go. As for why Zhu Yuanzhang had to kill Erhu and Yuel, there are mainly the following reasons.
The first reason is that Erhu and Yu'er had been following Zhu Yuanzhang and Empress Ma for too long, and knew too many secrets of Zhu Yuanzhang and Empress Ma. While Yu'er has been taking care of Empress Ma with no problem, Erhu is Zhu Yuanzhang's beloved, and Zhu Yuanzhang is responsible for a lot of things that are not convenient for him to do personally. So, Erhu followed Zhu Yuanzhang for so many years to know Zhu Yuanzhang's dirty things must also be very much, Zhu Yuanzhang think not to kill the two words may be those dirty things will be leaked out.
The second reason was that the two tigers had offended too many people, and Zhu Yuanzhang killed them to save them from being hunted down by their enemies, and to make their deaths more honorable. While it was said that many of the deaths and injuries were ordered by Zhu Yuanzhang, many times it was Erhu himself who led the teams to capture or kill them. Those people didn't dare to lay their hands on Zhu Yuanzhang, but they knew that Erhu and Yu'er would definitely retaliate against them after they left the palace. So Zhu Yuanzhang killing them could kind of salvage some political influence and prevent them from being insulted by their enemies.
The third reason is that the big tiger of that year is also counted because Zhu Yuanzhang died, Zhu Yuanzhang is afraid that one day Erhu will take revenge for the big tiger. Although the death of Little Ming Wang was Hu Weiyong's hint that Dahu did that, Zhu Yuanzhang let Dahu die in order to eliminate the influence. Although the two tigers followed Zhu Yuanzhang for many years, Zhu Yuanzhang and the two tigers knew how the big tiger died. Because Zhu Yuanzhang was afraid that Erhu would avenge the death of Dahu, he had to kill him as well.