Whiskey_Nick posted... Vader was hunting/stalking/tormenting him.
He wants Obi-Wan to suffer and saw that he is literally zero threat. Plus Star Wars is littered with people letting people get away when they should be easily attained. Like it happens A LOT.
I am certain we are getting another fight between them in the 6th episode where Obi-Wan is back to his normal power. That colony is gonna give him hope or some such.
yeah this is how I took it,
like as Vader was watching he definitely could have done something if he wanted but he was just caught in thought. To me it's that he's been looking to squash Obi-Wan for years and now that he's found him it's so unsatisfying to find this feeble state that he's in.