kick somebody when they're down
kick (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 kick him while he's down. A: "I'm sorry you got fired, but you shouldn't have been so sloppy with your work." B: "Wow, way to kick me when I'm down."