Although Padmé believed that there was still good in Anakin Skywalker after the end of the Star Wars prequel trilogy, Darth Vader's final form proves that she was wrong. Anakin was the Chosen One, conceived by the Force to bring about "ultimate balance," but that turned out to involve a lot more tragedy than anyone expected. Anakin's deep-rooted need for attachment ultimately led him to the dark side, and he became Darth Vader, the Emperor's Fist.
The Emperor manipulated Anakin Skywalker with tremendous skill, seducing him to the dark side with the promise of finding a way to save his beloved Padmé. In reality, Anakin's transformation into Darth Vader killed everything Padmé ever loved about him. Stung by what he believed to be Padmé's betrayal, Darth Vader used the Force against his wife, and she subsequently died in childbirth. Padmé died with hope still in her heart, however, insisting that there was still good in him. The Jedi never shared Padmé's hope, with Obi-Wan eventually expecting Luke Skywalker to kill Darth Vader. Luke chose a different path, instead deciding to believe Darth Vader could be redeemed, and he apparently proved Padmé right in the end.
Darth Vader #25, by Charles Soule, Giuseppe Camuncoli, and Cam Smith, suggests this is too simplistic an interpretation. The comic is essentially a mystical experience Darth Vader enters into in an attempt to resurrect Padmé, and it sees him reduced to an avatar in the Force - one that truly represents the state of his heart. Darth Vader is shown to be a being of churning darkness, a maelstrom of malevolent energy. While there is light around him, it is represented in the form of white butterflies desperately flurrying around him; these probably represent Padmé's love. Darth Vader #25 makes it clear there is no good in Darth Vader.
This fits perfectly with George Lucas' own interpretation of the Star Wars saga: that Obi-Wan Kenobi was right to say Darth Vader killed Anakin Skywalker, and that his redemption in Return of the Jedi is best viewed as a sort of resurrection. This is why he had Hayden Christensen inserted into the Special Editions, with Anakin's Force Ghost de-aged to his prequel era form. Anakin had died, but Luke's redemptive love brought him back. The light did not come from inside Darth Vader, but from outside - like those butterflies. But Luke's light accomplished what even Padmé's could not, entering into that maelstrom of darkness and transforming it.
Star Wars tie-ins have further revealed Darth Vader's attack on Palpatine was the only way the Emperor could be defeated. Palpatine had mastered the technique known as Essence Transfer, a power that allows a Sith to transfer their spirit into an object or even another person's body at the moment of their death. A potential host has to be under the influence of darker emotions to be vulnerable to possession, however, and Darth Vader acted out of love not hatred. Luke Skywalker's redemptive love truly transformed the Star Wars galaxy.