This is both a very good point and a very terrible analogy.

A better one would be that you see that Mr. A is brutally beating Ms. B.

What you don't know is that Ms. B just tried to run over Mr. A's son with her shopping cart. She only gave him a scrape, but she said when confronted that she'll keep trying every chance she gets until he's dead or in the hospital.

You don't need to know that she wants to run over his son because Mr. A's family swindled her family out of land, which he did because his family had been swindled out of that land long ago by someone else, and when he came with an offer to buy the land he got only insults and threats, etc. etc. etc..

But you do need to know that Mr. A was moved to violence because Ms. B committed violence against Mr. A's loved one. That is the simplest account needed to make a moral judgment, because "defense of loved ones against violence" is considered a morally valid reason to engage in violence. We might think Mr. A's actions are still worthy of censure (e.g. we might judge them excessive, we might judge that he had other better options than violence this time), but we can't judge sans context.

We can express horror at what is happening to Ms. B because we empathize with how unpleasant it is to be beaten, but we can't really know how to judge Mr. A without that key bit of context.