Masherbrum,
Well.. an easy trigger finger isn't a good thing for a cop.
Might get innocents injured or killed, or even fellow cops.
Guns should be only used as a last resort, regardless what the criminal had done.
Shooting at an incapacitated criminal with a shotgun is a little hard to explain with anything but to make sure the guy gets what he deserves.
The cops do not exist to be the judge and jury, they exist to maintain the law.
I don't think the law gave them the right to make sure the criminal is dead.
Otherwise they could just as well use a rocket to stop the fleeing criminals, would make it easier.
Or shoot the criminals on the spot, without questions asked.