Are there any English idioms to describe “futile benevolence?”

We have a word, “宋襄の仁” meaning “futile benevolence.” The word comes from a historic episode from ancient China.

In Spring and Autumn era (BC 8C) in China, when Song Country fought Chu Country, Muyi, Prince of Song besought his father, King Xiang to attack the enemy before the enemy completes deployment of their army, King Xiang rejected his son’s advice because it’s unfair to attack people when they are in trouble.

King Xian lost the war and Song was conquered by Chu. Hence the word, “宋襄の仁 – Sojo-no-jin - King Song Xiang's benevolence” passes as the futile benevolence or stupid generosity in our country, and possibly in China.

Are there any English phrases or idioms that are derived from similar episodes in which excessive or inconsiderate benevolence / generosity turns out to be a great disaster?

Based on the meaning and the description of the event, I don't think "futile benevolence" is a very good rendering into English of 宋襄の仁 (of the concept).

I think the corresponding concept is "heaping coals on the heads of your enemies" or being a true "Good Samaritan" since the action of Song Xiang was to help his enemies

Or if you're looking for a non-Scriptural equivalent, I might say blind chivalry

There is the proverb "no good deed goes unpunished."