A gunman casually executed a homeless person on a St. Louis street in broad daylight ... and the horrifying attack was all caught on video.
Check out this footage -- shot by a witness -- showing Deshawn Thomas brandishing a gun while standing just behind the homeless man outside a downtown building on Monday morning.
Thomas struggled at first to load the weapon as the homeless man sat on the edge of the sidewalk ... oblivious to the danger around him.
With the gun loaded, Thomas pointed the barrel at the head of the homeless man and pulled the trigger.
The witness reacted with sheer terror, screaming, "Oh my God, he just killed him!!"
Thomas fled on foot, but police later captured him and recovered his gun as he tried to enter a library near the crime scene. He was booked into the county jail for first-degree murder.
Cops say Thomas got into an argument with the victim at a gas station prior to the shooting. It was not clear what sparked the dispute.