Re: Good/evil, what's the difference?
Darth as far as in Imperial space would be Lawful Neutral, at the top. He doesn't use the Law (Imperial law) to do good, he is a Sith, who are all 'might makes right', so he'd err to evil just for that more than anything else. Yes, he does break alignment to save Luke and find redemption in Return of the Jedi, a final act that is 'Good' (he takes the Emperor's Force Lightning in defence of another, even if it is his son).
I won't go into the moral implications of what makes good or evil, but rather point out the split that doing Good alignment-wise generally means being charitable, helping people in a selfless way and generally being willing to sacrifice to help another. Evil acts are causing pain and damage for the lulz and doing acts which cause grief to others but better yourself because you can.
Is killing an act of good? Hm, not really as written, but killing someone or something that's been going about doing evil as written above, especially if that evil one has been killing others, that would probably be an act of good as you're doing an act to save others, such as killing a dragon as they've killing a bunch of people in a nearby village. That would be a good act to help the village. Long as you're not killing the dragon because of something like 'killing is awesome' in doing it, it'd be an act of good. Otherwise, you would be erring to an evil act justified as a good one.
Again, real life is much more complex, but thats a can of worms I'll stay away from.