【落井下石】

kick/hit sb. when he is down对人落井下石

add insult to injury雪上加霜

twist/turn the knife (in the wound)揭人疮疤

rub it in反复提及令人不快的事

add fuel to the flames/fire火上浇油

rub/grind salt in sb.’s wound在人伤口上擦盐

fan the flames火上加油

slap in the face侮辱

a stab in the back 暗箭伤人，背后捅刀

make groundless accusations 诽谤中伤，捕风捉影