An idiom for “striking unnecessarily hard when the opponent is already weakened”

In Assamese there is an idiom that means 'striking unnecessarily hard when the opponent is already weakened'. Is there any such idiom in English that could mean the same?

Solution 1:

There is 'kicking a man when he's down'

verb To kick a man when he's down is to attack at the persons weakest moment. It defies the gentlemanly code of ethics, and does detract from reputation. Used literally or figuratively, it still has pretty much the same meaning.

www.urbandictionary.com

Solution 2:

It's not an idiom, but perhaps the word overkill might work.

Overkill: Excessive use, treatment or action

Source: Oxford Dictionaries Online (http://www.oxforddictionaries.com/definition/english/overkill)

One of the usage examples on this page is

While it may seem like overkill, the military was finally satisfied.

Solution 3:

Perhaps:

To kick someone while they're down: Originating from the fighting ring where it's bad form to keep beating a person while they're down or out, it now also means to make things worse for someone who is going through a difficult time.

While not perfect, there's also:

Hitting below the belt: Striking out, either physically or verbally, in an unfair way.