It began in a way gun advocates have suggested would curtail violence.
A gun comes out. Shots are fired. A "good guy with a gun" steps in to help before police can respond.
The tidy theoretical doesn't account for the chaotic unknowns when police arrive and can't tell a "good guy" with a gun from a "bad guy" with a gun.
The theory turned to grim reality at Manny's Blue Room Bar in Robbins, Illinois, outside Chicago.
Police shot and killed the good guy. Jemel Roberson, 26, was working for security.
"Everybody was screaming out, 'He was a security guard,' and they basically saw a black man with a gun and killed him," witness Adam Harris told WGN.
The incident may become a touchstone in a persistent debate about how places such as schools, nightclubs and houses of worship should steel themselves against gunmen.
That debate has gained urgency during the past year, as US President Donald Trump and others have repeatedly said security guards - specifically armed ones - could have prevented the nation's mass shootings; earlier this year, Trump tweeted his support for the controversial idea of arming teachers.
And the Chicago incident has already provoked concerns that black men, even when legally carrying firearms or employed in a position that allows their use, can still become a target for police fire.
Roberson had a valid gun owner's licence but did not have a concealed carry permit, WGN reported. In Minnesota in 2016, Philando Castile was killed by an officer during a traffic stop seconds after he told an officer there was a weapon in the car.
Details about the Illinois shooting were unclear. Midlothian Police Chief Daniel Delaney said in a statement that officers from his department and the Robbins Police Department responding to a shooting found multiple victims inside the bar.
The statement said the "Midlothian officer encountered a subject with a gun" and shot him.
Delaney did not elaborate on what occurred before the police response.
The incident began with a confrontation involving several men, and a man left to retrieve a gun. He returned to the bar and opened fire, striking several people, the Chicago Tribune reported, citing remarks from Robbins Police Chief Roy Wells.
Roberson returned fire and apprehended one suspect, the Tribune reported, as officers from suburban police departments responded.
It is unclear how the killing unfolded from there - how and whether officers identified themselves, whether Roberson was holding a gun, or how much time passed before the officer fired at Roberson, who was pronounced dead at a hospital.
The Cook County Sheriff's Office and Robbins Police Department, neither of which responded to requests for comment, are investigating the shooting that first drew the police to the scene, Delaney said.
Illinois State Police will investigate Roberson's killing by the Midlothian officer. Delaney did not answer a question about the administrative status of the officer.
Roberson is one of at least 840 people who have been shot and killed by police so far in 2018 and one of at least 19 in Illinois, according to a Washington Post database.
At least 173 of those shot and killed by police this year - 21 per cent - were black. The US population is about 13 per cent black.
More than half of those killed - 459 people, including Roberson - were said to have a gun when police killed them.
Roberson was a musician who played drums and keyboards at churches in the area, Patricia Hill, a pastor at Chicago's Purposed Church, told WGN.
But Hill said Roberson had a greater ambition: To one day become a police officer.