That crossed my mind too at first that since they were retreating maybe he should not be pursuing. But, all they had to do is stop, turn around and shoot back at him. If they had dropped their guns, ok, but they were still armed and potentially life threatening. I'm not a lawyer, but the threat was not gone until they left the room, not while they were running away.
Keep life simple...if you hear hoofbeats, think horses, not zebras.