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, Ill., outside Chicago early Sunday.

Police shot and killed the good guy. Jemel Roberson, 26, was working 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.

I expect nothing but crickets from the NRA over this, because their underlying reason for existence is their advocacy for the ability of white people to shoot black people whenever they see fit.