In the video, a man rushes to the driver's side of a large white truck ... with what appears to be a gun in his hand, and he's screaming at the other motorist while punching his door during the Tuesday incident.

Watch closely ... because when he's rushing up to the car, the gun's in his right hand -- but, when he reaches the door he switches it to his left before raising his arm and pointing.

That's when the other driver points his own gun and fires 3 fatal shots.

The victim -- later identified by Indianapolis Police as Gavin Dasaur -- drops to the ground, clearly injured.

The Indianapolis Metropolitan Police Department tells TMZ ... officers responded to a call that a person had been shot after 8 PM Tuesday. When they arrived at the scene, they found Dasaur injured and transported him to a local hospital where he was pronounced dead.

Cops say they detained the shooter at the scene, but let him go after consulting with the Marion County Prosecutor’s Office. Local reports say they determined this might have been a case of self-defense, and released the alleged shooter.

Watch the video again, it seems to support the self-defense theory -- the guy inside the truck never sees Dasaur switch his weapon to his left hand, so he likely thought Dasaur was about to fire when he pointed his hand inside the truck.