The shooting happened in the Missouri street on Monday.
Deshawn Thomas, 23, has been arrested in connection with the shooting but he is yet to be charged, police said.
In the video, the alleged shooter can be seen standing a few feet behind the homeless man, who is sitting on the path, while he struggles to load his gun.
The man on the ground appears to see Thomas and raises his hand but made no attempt to run away, seemingly accepting his fate.
