stick the knife in

To be, do, or say something especially critical, unpleasant, or mean-spirited, especially to someone who is already vulnerable or weak. John seems genuinely remorseful for what happened. There doesn't seem to be any reason to stick the knife in at this point. And then he really stuck the knife in by telling me that he'd only pretended to like my writing.

put/stick the ˈknife in

put/stick the ˈknife into somebody

(informal) criticize somebody or deliberately try to harm them: The senator has quite a few enemies that would love to stick the knife in if they got the chance.