The President of the United States is caught making advances on an underage "Firefly Girl" less than two weeks before Election Day. Conrad Brean (De Niro), a top-notch spin doctor, is brought in to take the public's attention away from the scandal. He decides to construct a diversionary war with Albania, hoping the media will concentrate on this instead. Brean contacts Hollywood producer Stanley Motss (Hoffman) to create the war, complete with a theme song and fake film footage of a photogenic orphan (Dunst) in Albania.
When the CIA learns of the plot, they send Agent Young (Macy) to confront Brean who convinces him that revealing the deception is against his best interests. The CIA announces that the war has ended, but otherwise maintains the deception and the media begins to turn back to the President's abuse scandal. Motss decides to invent a hero who was left behind enemy lines, and inspired by the idea that he was "discarded like an old shoe" has the Pentagon provide him with a soldier named Schumann (Harrelson) around whom he constructs a further narrative including T-shirts, additional patriotic songs, and faux-grassroots demonstrations of patriotism. At each stage of the plan, Motss continually dismisses setbacks as "nothing" and compares them to past movie-making catastrophes he averted.
When the team goes to retrieve Schumann, they discover he is in fact a criminally insane Army prison convict before their plane crashes en route to Andrews Air Force Base. The team survives and is rescued by a farmer, but Schumann attempts to rape the farmer's daughter and the farmer kills him. Motss then stages an elaborate military funeral, claiming that Schumann died from wounds sustained during his rescue.
While watching a political talk show Motss gets frustrated that the media are crediting the president's win to a tired campaign slogan of "Don't change horses in mid-stream" rather than Motss's hard work. Despite previously claiming he was inspired by the challenge, Motss announces that he wants credit and will reveal his involvement, despite Brean's warning that he is "playing with his life". Motss refuses to back down, so Brean reluctantly has him killed and makes it look as if he had a heart attack. The president is successfully re-elected and a news report about a violent incident in Albania is shown, but it is ambiguous whether this is a true event or simply a continuation of the fictional war.

Question: Why does Brean have Motss killed?
Answer: He is threatening to reveal his involvement in order to receive credit for the cover up of the presidents scandal.
Alternative answer: Motss was going to reveal the President's involvement in the made-up war.