Use a comma to separate coordinate elements at the end of a sentence or to indicate a distinct shift at the end of a sentence.
This comma rule is pretty straightforward. If you have a distinct shift at the end in content or a shift at the end for emphasis, you should set off that shift at the end with a comma.
You want me to fight Chuck Norris for you? I am brave, not crazy.
You are going to stand in line for the new Halo 4, aren’t you?