10:21 AM PT -- Jemel's mother, Beatrice Roberson, filed a lawsuit Monday morning against the police officer who killed her son.

In the suit, she says the shooting was unprovoked, unjustified and "excessive and unreasonable."

She's suing the officer as well as the Village of Midlothian, and she's demanding damages in excess of $1 million.

A security guard who took down a man who opened fire in a bar was shot dead by a cop who responded to the 911 call.

Jemel Roberson was working security at Manny's Blue Room Bar Sunday at 4 AM in a Chicago suburb when several drunk patrons were ordered out. The guys left, came back with a gun and opened fire.

Roberson returned the fire and apprehended one of the men. Roberson placed his knee in the gunman's back and point a gun at him to stay still. An officer who responded to the 911 call fired at Roberson ... shooting him dead.

One of the bar patrons who witnessed the shooting said the cops "saw a black man with a gun, and basically killed him."

Roberson was a musician who played at several Chicago-area churches and wanted to become a police officer.

The Cook County Sheriff's Office has opened a criminal investigation into the bar shooting.

A GoFundMe account was established to pay for the 26-year-old Roberson's burial.