Dear LIB reader: My son is awfully mean to his brother

From a LIB reader

I have an 11 year old son who is awfully mean to his 8 year old brother. He never sees anything good in his younger brother, and is always either screaming at him or hitting him. His behaviour is so irrational that I'm scared of leaving them alone because of fear of what my 11 year old might do to the 8 year old when no one is there to stop him. Yes I am that scared!

He's nice to other people but always mean to his brother. Are there other mothers out there experiencing same? Is it normal for a young boy to detest his own brother? What could be the cause and what can I do as a mother?