My brother beats our father but is it deserved?

My father used to treated us differently. He was softer with me but he used a belt on my younger brother.

I still recalled that one time he overdid and my brother (then 3 years old; while I was 9) was crying loud, screaming, shaking and his eyes were very puffy. He had minor bruises on his bottom.

He's now 23 and has literally knocked out our father during an heated argument. This is the 3rd time he punched him and floored him. Now tablets are turned. My father is scared of him.

Is it deserved? My mom has no pity and says he's reaping what he saw. My brother still remembers that event and has resentment towards him and our mother for doing nothing.

Updates
8 mo
My brother doesn't hit women nor kids though. He has a very soft spot for them. He has no regret for knocking out his father.
My brother beats our father but is it deserved?
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