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. Yikes, are they friends or enemies? Because it sure seems like they take every opportunity to get the knife into each other.

get (or stick) the knife into (or in) someone

do something hostile or aggressive to someone. informal

get your ˈknife into somebody

have your ˈknife in somebody

(informal) harm and continue to harm somebody (usually not physically) whom you consider your enemy: He’s had his knife into me for months, and every time I make a mistake, he tells my boss.