Obi-Wan actually smiled and shook his head. "You truly do not know your master." It proved it. Not that he was surprised. Ben sighed and continued to walk as he explained. "Anakin Skywalker. The Stih you know as Vader. He was my apprentice."
There was sadness in his voice despite the smile on his face. He didn't look at Starkiller directly, though he did keep walking.
"I would have fought if I could, but I had other duties at the time." And while Obi-Wan was not one to cower, he knew himself. He'd likely made a promise. An important one that meant he had to stay near Luke long enough to protect him. And it seemed to have worked out in the end, hadn't it?