What strikes me about this is not so much the courage, but the kindness of the store clerk.
Mustafa comes at him with a gun, threatening his life for money. Derek knocks the robber out with a single punch and calls the police. But there is no more shouting or violence. Derek sits with him, almost in a comforting way, and gives him paper towels to help with the bleeding.
He has good advice to offer too: If you want money, get a job. Work, like everyone else.