Why does Sgt. Barnes shoot Sgt. Elias right in front of the camera crew?

I was watching this documentary about the Vietnam war called "Platoon" and it follows a platoon in 1967-1968 time. When I was watching it Sgt. Barnes tells Pvt. Taylor he is going to get Sgt. Elias but instead finds him with the cameraman and shoots him. Sgt. Barnes never got in trouble because he was killed in action by Pvt. Taylor. What is up with these people killing each other?