Darth Vader: Obi-Wan once thought as you do. You don't know the power of the dark side.
Yoda: When 900 years old you reach, look as good you will not.
Darth Vader: If you will not fight, then you will meet your destiny!
Luke Skywalker: Never. I'll never turn to the dark side. You failed, your highness. I am a Jedi, like my father before me.
The Emperor: If you cannot be turned, then you will be destroyed.
The Emperor: If you will not be turned, then you will be destroyed!
Han Solo: I have a bad feeling about this.
Luke Skywalker: You told me Vader betrayed and murdered my father.
Obi-Wan Kenobi: Your father was seduced by the dark side of the force: he ceased to be Anakin Skywalker and became Darth Vader; when that happened, the good man who was your father was destroyed. So, what I told you was true...from a certain point of view.
Darth Vader: You cannot hide forever, Luke.
Luke Skywalker: I will not fight you.
Darth Vader: Give yourself to the Dark Side. It is the only way to save your friends. Yes, your thoughts betray you. Your feelings for them are strong. Especially for...sister. So, you have a twin sister. Your feelings have now betrayed her too. Obi-Wan was wise to hide her from me. Now his failure is complete. If you will not turn to the Dark Side, then perhaps she will.
Luke Skywalker: [ignites lightsaber] NEVER!
Han Solo: I think my eyes are getting better - instead of a big dark blur I see a big light blur.
Answer: Darth: "Obi-Wan once thought as you do. You don't know the power of the dark side. I must obey my master." It's implied that the dark side is intoxicating, once you totally give in to the dark side it has a hold on you, and appeals to morality and rationalization are useless against your lust for its power. (On a symbolic level, the dark side is a metaphor for vice. Darth Vader is an addict and abusive parent. It's actually funny how many scenes still make sense if you replace "the dark side" with "the bottle" or similar).
TonyPH
If we bring the prequels into it, it's one of the criticisms of those films that they only make the question of how much Anakin is a "true believer" more confusing. But it stands to reason that at first Anakin may feel vindicated in his resentment toward the Jedi. Later on, Vader may not feel that as strongly, but by then his anger has turned toward himself for failing to save Padme. He may feel that a man as terrible as he does not deserve to be "rescued" from the dark side, leading to a feedback loop where he only gets further enamored with its power and does more evil things which causes him to hate himself even more, and so it goes.
TonyPH