get the knife into someone
get (one's)/the knife into (someone)
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 get the knife into him at this point. And then he really got his knife into me by saying that he'd only pretended to like my writing.