...that was attacking his daughter.   The narrative below strongly implies he was off-duty, but does not clearly say so.    

   This may or may not be worth watching.  The preview showed someone inserting a magazine into what appeared to be an AR, but this was most likely staged footage and the guy may not have used an AR to shoot the dog.  
   

TUESDAY, DECEMBER 4 - 10:00 PM  ET
Interactive Live Vote  
A child is viciously attacked by a neighbor’s dog. When authorities fail to act, her father -- a police officer -- takes matters into his own hands, kills the animal, and is charged with a crime. Is he a father protecting his family or a trigger-happy cop? That was the question a jury had to decide. What would you decide? Tell us in a special interactive Dateline.

Viscious dog munching on my family member or attack other person and refuse to go away WILL be DEAD dog on the spot.