Luke had no intention of killing either; he was moving to defend himself and look for a way to stop Kylo without having to resort to fatal measures. He knew that he could likely have beaten him easily at full strength, but this wasn't the time or place for going all out.
He could easily see who Kylo was mimicking. He was acting like Vader, right down to a move similar to a Force-choke, but that didn't cut off his oxygen. It just held him still, and tried to break through his defenses, to call up his fears and his pain. Luke braced himself, fighting it. Luke felt his thoughts about Vader, even though he wasn't actively entering his mind. Kylo seemed to want to be just like him.
The woman as he now realized the third person was fired at Kylo, and Luke saw what she was wanting. Kylo would have to deflect the bolt, and it would allow Luke to back away or help diffuse the conflict.
< I never meant for such trouble. > he told her through the Force, shielding from Kylo and not taking his eyes off him. < I tried to avoid a battle...I did follow the darkness I felt here, but I have been hoping that I might help him as I helped my father. It seems I've just upset him more. >