After Sonny's brother reads in the paper about Sonny's arrest, and an old friend of Sonny's meets him in the school parking lot, he is reminded of how he has given up; how he made a promise to their mother; how he feels helpless; and he sees someone else express the same despair about helping Sonny. The brother's response to all these memories, to Sonny's arrest, to the friend's remarks is one of anger, but his anger is laced with despair and probably desperateness at his helplessness.
It has been a year between the last time the brothers met and Sonny's arrest. The incident with the old friend in the parking made Sonny's brother recall a conversation with his mother during which she told him about their father's brother who was gunned down by drunken whites, but it was an object lesson to impress him with the truth that as brothers, they rely on each other. She extracted the promise that, as the older and more steady of the two brothers, he would always look after Sonny and help him. It is these recollections and the feelings of shock and horror at his brother's arrest coupled with helplessness that cause him to lash out in anger at the friend.