Another witness says the suspect was just trying to hold his pants up as he ran away.
As an aside, at the Lakewood WA shooting of 4 police officers one of the officers made contact with the suspect and had grabbed his gun when the suspect drew a second gun from the rear waistband of his pants and then made a contact shot on the officer's head. He then placed another contact shot through the officer's eye socket after the officer was down.
Garner vs. Tennessee is the court decision which lays out the precedence for shooting at fleeing suspects. The suspect needs to pose a credible threat to either officers or the public to be a justifiable shoot. Personally, a guy that wants to fight cops and tries to pull a gun on a cop probably poses a danger to the citizenry and definitely other cops.
Sounds like any given episode of "Cops".