The Dark Side was giving him power, but he was so overwhelmed with the emotions and lack of control, he could never have killed Obi-Wan in that duel.
Also, the RotS novel also states Vader had chances of killing him to, it wasn't just a one way street, Obi-Wan wasn't holding back, Vader wasn't holding back, there were simply pauses, as in all duels, Vader was too arrogant, too uncontrollable, Obi-Wan had every advantage he needed to hold him off.
Just look at Anakin and Obi-Wan vs Dooku II, straight away, he dispatched Kenobi, at this point he was messing around, when Anakin changed forms(they were trying to trick Dooku into a false sense of security) to Djem-So, Dooku didn't stand a chance, he knew Anakin was the best Djem-So user, he knew all of his decades of training and mastery of Makashi, were no match.
Vader had chances of killign him and tried to use those but Obi-wan swatted them, Vader's physical force almost crushed Obi's bones. Obi-wan literally had 2-3 times of killing him but he paused and there were all those paragraphs about Anakin being his brother and what not.
If you'd prefer me to pull out the novel, I can.