If you believe he was armed with your weapon?
Even allowing for the BS speculation that Slager was able to reload his tazer, according to this blog it had been fired again into the officer. So even if the suspect had the tazer he had no way to reload it and no way to be a threat. So no, there was no justification for shooting a fleeing, unarmed suspect in the back.
Then why did the cop move the taser and drop it near the body. That is evidence that he knew it was a bad shoot.