I like the growing consensus that Vader let him get away he was disgusted by the broken pathetic Jedi
That happened to Captain America once in the comics, IIRC. Cap was distracted and upset about something, a villain appears - I forget exactly who- and says [along the lines of] "Yeah old man, I could take you now but Im not doing that, because I want you at the top of your game, I want to prove to everyone you ain't really all that, and killing you now would be a cheat. Right now anyone could kick your @ss. I'll be back later when you're over all this and I'll be bringing an @ss-beating with me when I do" and Cap was thinking to himself, "Great!...!"
I could see Vader doing this.