hit somebody when they're down
hit (one) when (one) is down
To criticize, exploit, insult, or otherwise treat badly someone who has already suffered a setback or is in a vulnerable position. Can Jeff's employee review wait until next week? His girlfriend just left him, and I don't want to hit him while he's down. Bob: "I'm sorry you got fired, but you shouldn't have been so sloppy with your work." Dave: "Wow, Bob. Way to hit me when I'm down."