The bloke screams at him that he is “not German, he’s a w***er”.
It was apparently after the suspect walked across the carpark roof shouting: “I am a German!”
But the bystander tells him he is a “w***er” and an “arsehole” as he urges him to drop his weapon.
A 32-year-old resident of Riesstraße witnessed one of the attackers run into the shopping centre from the balcony of his flat.
He claims the suspect – who was wearing dark trousers, a dark t-shirt and carrying a red backpack – was clutching what he thinks was a pistol.
It has emerged the gunman may have lured children to their deaths with a fake Facebook post offering free meals at McDonalds.
The chlling detail was revealed in a press conference in the wake of the attack which left 10 dead.
A total of 16 others were injured, three of them critically, and police confirmed some of those killed were children.
His body was found in a park next to the shopping centre and police are currently checking his rucksack for explosives.
Cops have confirmed he was the only gunman, despite initial claims there were three armed lunatics on the loose.