During the Spring and Autumn period, the states of Wu and Yue were locked in constant warfare.
In 494 BCE, King Fuchai of Wu crushed King Goujian of Yue's army at Fujiao (in present-day Taihu Lake, southwest of Wuxian County, Jiangsu), then pressed his advantage and stormed the Yue capital.
After his defeat, King Goujian of Yue fled with his remaining troops to Mount Kuaiji
Wu Zixu, a senior minister of the Wu Kingdom, urgently advised the king, "I have heard that cultivating virtue requires constant nurturing, but eradicating evil must be thorough and complete. The Yue Kingdom has shared this land with us for generations and has always been our sworn enemy. If we accept their surrender without destroying them, they will surely seek revenge one day—and by then, our regrets will be too late."
But Fuchai ignored Wu Zixu's advice and accepted Goujian's surrender. Wu Zixu sighed, "Alas! One day Wu will surely fall to Yue!"
King Fuchai of Wu captured King Goujian of Yue and his followers, bringing them to the Wu capital, where he first forced Goujian to tend graves and herd horses, and later released him back to his own kingdom.
After returning to his kingdom, Goujian slept on brushwood and tasted gall, fueling his determination for revenge. He appointed wise ministers like Fan Li and Wen Zhong, and after twenty years of relentless effort, he transformed his weakened state into a powerful force, ultimately launching a campaign that destroyed the Wu kingdom and avenged his humiliation.
King Fuchai of Wu "failed to eliminate evil completely," and as a result, he was defeated in battle and forced to commit suicide, reaping what he had sown. Later, the idiom "eliminate evil completely" came to describe the necessity of eradicating evil forces thoroughly and without mercy.
Source: *Zuo Zhuan*, "First Year of Duke Ai"
Meaning of the Idiom: Later, the Chinese idiom "除恶务尽" came to describe how the necessity of eradicating evil forces thoroughly and without mercy.