To take a line from the ROTJ novelisation, something which has stuck with me through the years, and forgive the paraphrasing, "Vader did not hate, he just lusted too blindly". Whereas in an interview with the official Star Wars magazine some time ago, Ian McDiarmid compared Vader to Lucifer Morningstar, insofar as he is a fallen angel, whereas Sidious represents original sin; he has no redeeming qualities and is evil for evil's sake. His only saving grace is that he is a patron of the arts, as seen by his visit to the opera.
In ROTJ, there is a definite sense that Luke is teetering on the edge of darkness, after starting off so promisingly in A New Hope. Anakin, meanwhile, is following a road that seems inevitable from the start (okay, we as an audience know what his fate will be but even if we didn't, the seeds of his destruction are there from the get go - he has already formed an attachment to his mother and his fear of losing her is too great to overcome). As with all things Star Wars, the story is better than the execution. I have found this out in telling people who are neither sci-fi nor fantasy fans what the story of Star Wars is about. They've been enraptured by the telling (and I'm no Stephen King) but when they've sat down to watch the films, their attention has drifted somewhat.
What's interesting is the parallels in Luke and Anakin's journey. When faced witht he death of his aunt and uncle, Luke firms his resolve to join the rebellion; when Anakin loses anyone close to him, he goes on a murder spree.
I'm probably in the minority here - this site is called originaltrilogyform after all - in saying that I think both prequels deal with the journey to the dark side in equally successful ways.