hit (one) when (one) is down

To criticize, exploit, insult, or otherwise treat badly one 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." A: "Why do you keep giving Todd money?" B: "Because he's my little brother, and I know he's struggling. How can I hit him when he's down?"

hit somebody when they’re ˈdown

continue to hurt somebody when they are already defeated: You wouldn’t hit a man when he’s down, would you?