Justice Kevin Mullins was seated at his desk inside his chambers in the Letcher County Courthouse in Whitesburg, KY, on September 19, when Sheriff Shawn "Mickey" Stines walks inside and appears to pull out a gun and shoot him multiple times, killing the judge.

Stines surrendered to authorities and was charged with Mullins' slaying. Investigators uncovered a possible motive, revealing Stines allegedly found his daughter's phone number on Mullins' phone and called it -- and a detective later confirmed the sheriff's daughter had been called from the judge’s phone before this incident.

A security camera recorded the judge's shooting death, and the clip was played during a preliminary hearing in the courthouse.

Check it out ... Mullins is seated in his desk chair with his hands up, seemingly trying to talk sense to Stines, who's standing feet away holding a pistol at his side.

Suddenly, Stines appears to point the gun at Mullins, who turns away in fear. Moments later, Stines allegedly pumps Mullins full of bullets, circling the desk and firing multiple times at the judge lying on the ground ... in the full-length video shown in court.

Considering the video evidence, Stines certainly has an uphill battle convincing a jury he's innocent ... but he's pled not guilty anyway.