A walker noticed the two men, aged, 25 and 31, with a gun on Monday at 7.50pm in a beer garden in Garching, a suburb north of Munich. He called the police and when officers arrived and shone their torches at the brothers, one of the men raised the gun to his head and fired a shot.
Police could only look on as the second brother then grabbed the 9mm Heckler & Koch gun and did the same.
The beer garden was closed at the time due to bad weather.
Police said it was not yet clear if the two brothers had taken drugs before the shooting, but they added the men were known to police for offences including drug trafficking as well as theft.
The older brother, who lived with his grandmother, had been treated for suicidal tendencies by a psychiatrist.