You're assigning a motive without any evidence. The only logical motive was that he deemed the person to still be a threat, which would be a logical assumption.
You make the same assignment, LOL. I have video evidence clearly showing that he most certainly did NOT feel like the kid in white was a threat for over 38 seconds. He turned his back on him to leave the store, left his coworkers alone with him for 15 seconds, turned his back on him while going to the counter, didn't turn to face him while re-loading, and walked straight at him before firing the final shots. He did NOT take a place of cover to do those last shots, either. There are SIX clear indicators that this man did NOT feel a threat. from the kid in white after his initial shots. Argue a logical reason for THOSE and I shall concede your point.