(Vader was always Anakin Skywalker, therefore I will call Vader by his true name: Anakin)
Clearly, I am the person that could write a book about this as Anakin's character is very personal to me.
To sum it up Anakin Skywalker's story is a tragedy. He thought that losing those he loved was directly due to his inability to save them (his lack of power). Furthermore, the Republic with all its flaws was incapable of saving lives and making things better in his eyes. The inconsistency and truthfully the corruption of both the Jedi and the Senate was more evidence to Anakin that something completely different was needed to make things better. He was persuaded by Palpatine who understood the dark side of the force better than any of his Jedi mentors understood the light side that the only way to true power (and through it the control that Anakin wanted) was the dark side.
It is important to realize that to Anakin power was a means to an end. Whereas with Palpatine power was everything. I firmly believe that had the Empire been able to force the galaxy into "peace" that Palpatine would not have stopped looking for more power. This would create a conflict that would either lead to Palpatine having to find a new apprentice (as we saw him attempt in episode six) or Anakin killing Palpatine after realizing he wasn't interested in the "means to an end" version of power that Anakin was.
However, as the story goes we all know that Palpatine made the same mistake that Windu did. He tried to kill someone that Anakin cared about. Though, it was worse than Windu trying to kill Palpatine. This time it was Palpatine (someone Anakin's true self still knew to be bad) trying to kill one of the last pieces of Anakin's true self. Not only was it his and Padme's only son but it was the last person other than himself that knew that Anakin was still in there, still alive. Luke had not given up on Anakin even though he was close to giving up on himself. Yet this is where my views likely differ than most. I think that Anakin did not want to give up on himself. He did not want Luke to come to the dark side and give up to the temptation as he had. He truly wanted to be saved from being Vader. All he needed to finally save himself was Luke. It is just unfortunate that he couldn't do it himself. Though I disagree with a large portion of JamesSand's post I do agree that Anakin never took ownership of his own life. Not until the point that someone helped him believe he could. No one ever truly freed him (until Luke). That's the tragedy of Anakin.
To easily understand Anakin's view of the Dark Side I think that you should watch the movie "Hero".
SPOILERS: It is about the King of Qin (Qin Shi Huang) who became the first Emperor of China. He realized that the only way to bring peace to China was to take it all over and rule absolutely. There is much debate over whether he was right but historically in the time period after the war, China was brought into an era of peace unlike it had ever seen before. Of course during the war most hated him because the war was a destructive and deadly one. Yet, history shows that to the Emperor it was all a means to an end. All he ever wanted was peace for China but a massive war unlike any before it was needed to get there. A very interesting thought indeed and one that I think Anakin decided was the truth for his "land" as well. In fact, we know that Lucas took much influence from Asian culture and history to formulate Star Wars, so it would surprise me if this story was not one he drew inspiration from.
Anyways, that's a quick view of my understanding of Anakin. At some point, I think I will write something on my Journal about my views on this topic as it is very special to me. Thanks for reading.
