Detectives say the suspect, identified as 36-year-old Bongkuk Pak, was shot twice and was pronounced dead at a local hospital.
Clark County coroner's officials say Pak died of gunshot wounds to the torso.
Metro Police Lt. Les Lane tells the Las Vegas-Journal the sword Pak had was at least three-feet long.
Authorities say the shooting appears to have been in self-defense but that detectives were investigating whether the gun used by the Dairy Queen worker was properly registered. The clerk has not been arrested.
They say only two employees and no customers were present at the time of the shooting.
CBS affiliate KLAS reported Monday that a female suspect has been arrested in the case.
Metro Police arrested 47-year-old Carol Matteo, who faces charges of robbery with a deadly weapon, and conspiracy to commit robbery and burglary.According to KLAS,police say Matteo dropped off Pak for his unsuccessful, and ultimately, fatal, robbery attempt.