It was just before 8 pm on Saturday that the 49-year-old man stepped into a petrol station in the town of Idar-Oberstein in western Germany.
The man was to buy a six-pack of beer, but was refused by the treasurer because he did not have a mouth guard.
Throughout Germany, mouth protection is mandatory in public places such as shops and on public transport or where it is not possible to keep your distance.
Submitted himself
The man left the gas station and returned just over an hour later with a mouth guard.
But when he arrived at the cash register, he took off his mask, pulled out a weapon and shot the 20-year-old cashier.
On Sunday morning, the 49-year-old chose to submit himself.
He is now in custody on suspicion of murder.
According to Chief Prosecutor Kai Fuhrmann, the man said that anger grew inside him when he was refused to buy beer.
In his stress over the pandemic and his opposition to the demand for mouth protection, he saw no other way out than to "set an example", says the prosecutor according to Bild.