Secondly, in my opinion, this is also a necessity of Yongzheng. You know, the late Nian Gengyao had no fear of Yongzheng. He relied on his own merits, and then he felt that Yongzheng dared not touch him, and then he did many disrespectful things to Yongzheng.
For example, once he came back from the front, Yongzheng invited him to have dinner with his sister Hua Fei. At the banquet, Yongzheng made a delicious food for Nian Gengyao, but according to the etiquette at that time, this food needed to be treated by servants before it could be eaten. But there were not many people present at that time, just a few eunuchs and maids-in-waiting, one of whom was Yongzheng's great-grandfather Su Peisheng.
So Nian Gengyao spoke at this time and asked Su Peisheng to handle the food for him. At that time, Yongzheng paused when he heard this request, but he didn't say anything on the spot. He called Su Peisheng to carry out the general's orders, but Huafei, who was present, saw some clues and hurriedly knelt down to ask Yongzheng's forgiveness. Later, Nian Gengyao also made some explanations, but it didn't help in Yongzheng's heart. Through this incident, Nian Gengyao was included in his killing list.
In addition, Nian Gengyao was very arrogant in his later period, and no one could control him in that place in the northwest, which led many people to write letters to impeach Nian Gengyao. In this case, Yongzheng called Nian Gengyao back from the front, and then decisively put him to death.