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.
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I am Nick. Go Sens, Bills, Blue Jays, RedBlacks! UotY 2015, You should listen to The Show w/ Ngamer and Yoblazer