During the Spring and Autumn period, Duke Wu of Zheng married Jiang from the state of Shen. Jiang gave birth to two sons: the eldest, Wusheng, and the younger, Gongshu Duan. Wusheng's birth was difficult, frightening Jiang, so she never favored him, instead doting on her younger son, Gongshu Duan.
Lady Jiang repeatedly urged Duke Wu of Zheng to depose the eldest son and name her younger son, Gongshu Duan, as heir, but the duke refused. After Duke Wu's death, Wusheng ascended the throne, becoming known historically as Duke Zhuang of Zheng.
After Duke Zhuang of Zheng ascended the throne, he granted the land of Jing (present-day Xingyang County, Henan) to his younger brother Gongshu Duan, following the wishes of their mother, Lady Jiang. Gongshu Duan was thus known as the Grand Prince of Jing.
After Gongsun Duan arrived at Jing, he built its city walls larger than those of the capital. Minister Ji Zhong warned Duke Zhuang of Zheng, "According to regulations, the walls of a fief must not exceed those of the capital. Now, Jing's walls violate this rule, and disaster will surely follow."
Duke Zhuang said, "My mother, Lady Jiang, insisted on giving Jing to him. What can I do?" Zhai Zhong added, "Lady Jiang dotes on Gongshu Duan so much that it will surely lead to disaster. Your Majesty must prevent trouble before it arises!" The duke replied dismissively, "One who commits many wrongs will bring ruin upon himself. Just wait and see!"
Soon after, Gongshu Duan secretly annexed several cities along the western border, forcing them to pledge allegiance to him. When Minister Gongzi Lu learned of this, he confronted Duke Zhuang: "A state cannot have two rulers. Now the Grand Uncle in the capital is overstepping his bounds with such ambition—what do you intend to do, my lord? If you wish to yield the throne to him, allow me to go serve him. Otherwise, you must eliminate him!"
"Don't rush! He'll reap what he sows!" Duke Zhuang said. "The faster he expands, the sooner he'll collapse."
Some time later, Gongshu Duan expanded his army, stockpiled ample grain, and prepared to launch a rebellion against the capital, with his mother Lady Jiang secretly plotting as an inside ally, hoping to overthrow Duke Zhuang through a coordinated attack from within and without.
Duke Zhuang of Zheng learned the exact date of Gongshu Duan's rebellion and gathered solid evidence of his treason. Without hesitation, he ordered Prince Lü to lead two hundred chariots and launch an attack against Gongshu Duan.
Gongshu Duan could not hold his ground and was forced to flee the capital city
Lady Jiang feared that Duke Zhuang would punish her, but he forgave her, and mother and son were finally reconciled. Later, the idiom "He who is unjust is doomed to destruction" came to describe those who commit many wrongdoings and inevitably bring about their own ruin.
Source: *Zuo Zhuan*, "The First Year of Duke Yin"
Meaning of the Idiom: Later, the Chinese idiom "多行不义必自毙" came to describe how those who commit many wrongdoings and inevitably bring about their own ruin.