On chapter 29 Why is John so angry with his brother?

Stephen tells John that he has heard dangerous things, and that John is being watched. Perhaps, he adds, a friend might have been sent to the shop to deceive him. John laments having such a friend, then Stephen adds that Absalom had friends like that. John orders his brother out of the shop, and kicks over the table in front of him. Stephen has to leave the shop, and John locks him out. Stephen is humiliated and ashamed because he did not come for this purpose, but only to tell his brother that power corrupts and that a man who fights for justice must himself be cleansed and purified.