Or is it a Professional Hit ? Is this correct police protocol ?
No idea.
I guess if somebody is pointing a gun at the police and refuses to drop the weapon, when ordered to, it may be justified. But I don't think anybody is claiming he was pointing his weapon at anybody. It sounds like the police were shouting conflicting commands at the guy. One of the commands, according to some witnesses, was to "drop it". If he was not holding the gun when ordered to drop it then he may have started removing the gun from the holster in order to comply. Maybe that's when the cops started firing.
It could be that one or more of the cops is trigger happy. Once this cop started shooting the rest may have reacted out of panic. Because they refuse to release the video and audio we may never know.