strike a person when he is down 落井下石

“落井下石”，汉语成语，意思是看见人要掉进陷阱里，不伸手救他，反而又扔下石头（drop stones on somebody who has fallen into a well）；比喻乘人有危难时加以陷害。与英文习语“kick/hit/strike a person when he is down”意思相近。

例句：

我终是无法做到在吉米倒霉时落井下石。

In the end I just couldn't kick Jimmy when he was down.