Gai Xun, styled Yuangu, was a man of integrity and exceptional talent from Guangzhi in Dunhuang Commandery (present-day southwest of Anxi County, Gansu) during the Eastern Han Dynasty. Recommended as a "Filial and Incorrupt" candidate, he became Chief Clerk of Hanyang Commandery.
At that time, Hanyang was under the jurisdiction of Liang Province, and the provincial governor Liang Hu was a close friend of Gai Xun. Whenever Liang Hu encountered administrative difficulties, he would often seek Gai Xun's advice.
The Liang Province governed not only Hanyang but also several commanderies including Wuwei and Tianshui, where officials were notoriously corrupt and lawless. The Governor of Wuwei, relying on his powerful connections at court, tyrannized the people with impunity, sparking widespread resentment and hatred among the populace.
Liang Hu had a subordinate named Su Zhenghe, an upright official who feared no tyrant. After uncovering the crimes of the Governor of Wuwei, Su Zhenghe prosecuted him according to law, crushing the arrogance of corrupt officials.
Liang Hu, learning of this, feared that Su Zhenghe's actions would offend the Wuyi Governor's powerful patron in the court and implicate him, so he considered killing Su Zhenghe to protect his own position, yet he worried that such a murder would tarnish his official reputation, leaving him utterly indecisive.
Liang Hu then traveled to Hanyang, seeking advice from Gai Xun on how to proceed.
It happened that Gai Xun and Su Zhenghe were bitter enemies. Someone tipped him off that the provincial governor was about to consult him on how to deal with Su Zhenghe, urging him to use the opportunity for personal revenge and persuade the governor to have Su killed. Gai Xun flatly refused and said with righteous severity:
"To kill a virtuous person out of personal grievance is disloyal. To harm someone while they are in distress is unkind! Though Su Zhenghe is my adversary, I will never take advantage of his peril to strike him down."
A few days later, Liang Guo indeed arrived in Hanyang and asked Gai Xun how to deal with Su Zhenghe. Gai Xun said, "You raise and train falcons to be fierce so they can catch prey; but if you train them well and then kill them, what is the point of raising falcons?"
Liang Hu understood Gai Xun's meaning and abandoned his plan to kill Su Zhenghe. When Su later learned of this, he was deeply grateful and personally visited to thank Gai Xun, but Gai avoided meeting him. Gai told others, "I advised Liang Hu not to kill Su Zhenghe purely as a matter of official duty; it had nothing to do with my personal grievances with him!"
Later, the idiom "Taking Advantage of Others' Peril" came to describe threatening or attacking someone when they are in danger.
Source: *Book of the Later Han*, "Biography of Gai Xun"
Meaning of the Idiom: Later, the Chinese idiom "乘人之危" came to describe how threatening or attacking someone when they are in danger.