有仇不报非君子，有冤不伸枉为人
yǒu​chóu​bù​bào​fēi​jūn​zǐ, yǒu​yuān​bù​shēn​wǎng​wéi​rén

vengeance principle

有仇不报非君子，有冤不伸枉为人 (有仇不報非君子，有冤不伸枉為人) yǒu​chóu​bù​bào​fēi​jūn​zǐ, yǒu​yuān​bù​shēn​wǎng​wéi​rén
vengeance principle
one who doesn't avenge an injustice is not a gentleman, one who doesn't redress a wrong is not a man

Sources
Definitions
CC-CEDICT