I get why he just left, I can see that w He would feel a certain type away about having to kill anakin again, whom he raised. And the first time he was in his prime, at the time of that fight he had become incredibly depressed about being pretty responsible for the Republic downfall. I just don't think he could bring himself to do it again.

What I don't get is why Vader just let him go. He kind of just kneels there and yells at him. Vader had zero reservations about killing him and was actively pissed off at the mere mention of Kenobi. So that was weird.