The Grim Reaper is the person who killed her son, probably a terrorist.
The person who comes to bring the bad news and express condolences is a representative of our Armed Forces, a human being with a terrible task, but NOT the person responsible for the death.
I'm sure he doesn't like her calling him the Grim Reaper. And I don't have to have sympathy for a person who makes up mean-spirited names for her fellow human beings like this.
More as it becomes available from the scene at Lafayette Park.