宋襄之仁

Etymology
From the story of the Battle of Hongshui between the state of and  in 638 BCE, in which, in order to display his benevolence, waited for the Chu army to fully cross the river and rearrange itself into a battle formation before attacking it, leading to the thorough defeat of the Song army.

Idiom

 * 1) the ridiculous act of showing benevolence towards one's enemy