With the exception of Darth Vader, I can't comment on the cases you mention, because I have not read them.
However, one thing I have to say is that I think the number of people someone kills is a pretty poor indicator of how bad they are. Evil is, fundamentally, a state of the mind, and therefore of qualitative rather than quantitative nature. Sure, crimes and generally wicked acts can prove that someone is evil, but they are not the evil itself.
Here, motivation and circumstances under which the acts are commited come into the picture. Killing for a purely sadistic motive, i.e. simply because one takes pleasure in the act of ending the life of another human being, is the lowest motivation, pure and simple. Yes, I even rank it below greed, quest for greater power, vengeance or whatever other dark motive you can think of. And you can look at Ros...
To further illustrate why I feel the number of people someone has killed is a bad indicator of how evil they are, let us compare Joffrey's murder of Ros to
Magneto's turning of the missiles on the human fleets in X-men: First Class. Magneto was stopped in the last moment, but that is not the point, as he intended to blow up the fleets, which would have killed thousands.
Here is the thing, I still don't think he was as bad as Joffrey was when murdering Ros. The fleets had just backstabbed the mutants on the shore by attempting to take them out, without warning. While it may not fully justify Magneto's retaliation, he has a quite strong and understandable reason to be indignant and furious. Heck, I am even doubting it could be classified as war crime, technically. I am no jurist, but I don't think any military force attemping to wipe someone out is protected by the rules of war. I think Magneto should have been acquitted, in a neutral war tribunal, for that act.
The thing is that excessive retaliation is not ok, at all, but it is not as low a motivation as sadistic pleasure killing.
As for Darth Vader, I don't think he was sadistic, either. He was merely twisted in the mind by the Emperor. Yes, he did terrible things, including the murder of the Jedi children, backstabbing Mace Windu instead of helping him destroy the Emperor (an act which would likely have saved the Jedi order, including said children), choking his own wife to near death etc. So when he burned on Mustafar, which was intended to be tragic, I found that...just.
On the other hand, the Emperor himself was even more evil. Also, as for killing billions, I think Darth Vader only did so indirectly. It was Tarkin who ordered Alderaan destroyed (and he surely had the Emperor's permission to do so). Then, of course, we have the unnamed superlaser operator who pulled the button. A few people have a good share of the blame for all those deaths, but no matter what, clearly Vader can't shoulder all that it alone.
But yes, I think some people sympathize too much with lowlife scumbag character. If such a character is well-written, I guess they could be enjoyable to read about, but it is not like their succumbing to obstacles would bother me.