We report a patient with dementia due to B-12 deficiency and syphilis who presented to a forensic hospital after killing his ex-wife with a gun. Despite current awareness on the occurrence of aggression and violence in patients with dementia, there has been no report discussing dementia secondary to an infectious or nutritional origin causing homicide or severe violent behavior. We discuss possible mechanisms and several predisposing factors for violent behavior in the elderly. We also discuss use and access of a gun in demented patients and its complications. We recommend availability of neuropsychiatric assessments in the elderly, limitation of gun access to demented patients and inquiry about weapon possession in the elderly.