kick sb when he's down

kick somebody when he's down (hurt someone after he has fallen or failed) — бить лежачего; усугублять и без того тяжелое положение; добивать (перен.)

Example: If I make major mistakes on a project, the boss might come to me and ask me why I’m such an idiot. I’m already upset
that I made mistakes. He is kicking me when I’m down.

see also
[to add insult to injury]