You know this kind of makes me think of the difference between the end of Rogue One and the beginning of A New Hope. In Rogue One, Vader was merciless when he was trying to recover the stolen plans before the rebels left. In A New Hope, he seemed rather content to just let the stormtroopers deal with the rebels. But this was immediately after Rogue One, so why the change of heart? Maybe he was thinking, "I'm gettin' too old for this ****."