Before the film starts there may have been a struggle with the cop and the deceased over the taser or the officer's gun, if he had gotten shot during the struggle I could have accepted that this was self defense. Shooting him as he ran away, on the other hand is hard to justify... They could have chased him or captured him later. You shoot someone because they are a threat, not because you don't feel like running after them.
The fact that the cop originally lied on his report of how things went is not justifiable by any means and the fact that he moved the taser? to the body, apparently at first trying to make it look like the struggle had been there, or that the victim had been armed is just low.