Much of what Vader does in Episodes V and VI is him showing compassion in a twisted way, mainly in Luke’s interest. He convinces the Emperor to let him bring in Luke and attempt to turn him, rather than killing him outright. And when he does actually go after him, he doesn’t actually kill him, and in fact lets him go. In RotJ, Luke allows himself to be captured, knowing full well that Vader could kill him on the spot, but Vader doesn’t.
