Hit somebody when they’re down
Definition 1 of 1
Pro Tip 1/3Figurative Meaning
The phrase is often used metaphorically, not describing physical action but rather mean behavior. 
The media tends to hit celebrities when they're down by spreading negative stories.
Pro Tip 2/3Negative Connotation
Using this phrase almost always implies a negative judgment of the person acting. 
That politician was really hitting his opponent when he pointed out his recent divorce.
Pro Tip 3/3Emotional Weight
The phrase can evoke sympathy for the person being hit and disapproval towards the aggressor. 
Everyone felt sorry for Jake; the team hit him when he was down about losing the game.