The Jedi Council didn't fully agree with Qui-Gon but they couldn't deny Anakin was powerful with the force. Because of this, Anakin won his freedom and left Tatooine to become a Jedi. When Qui-Gon was killed in battle, his apprentice Obi-Wan Kenobi vowed to train Anakin against the wisdom of the Jedi Council. During this time period, Anakin met Queen Padme Amidala who he quickly fell in love with. As Anakin grew into a man they were secretly married, breaking Jedi Law.
As Anakin became a Jedi Knight he proved to be one of the most powerful Jedi ever and quite possibly the most ambitious. During the Clone Wars he formed a fatherly relationship with Chancellor Palpatine. Unknown to anyone, Palpatine was actually the Dark Lord of the Sith and was manipulating Anakin to become his apprentice. When Anakin was given a seat on the Jedi Council but kept from becoming a Master he became very confused and resentful. At this point Palpatine saw he was very close to turning Anakin. his promise to be the only one who could save Padme from the death Anakin was constantly having nightmares about finally pushed Anakin over the edge and to the dark side. Anakin quickly helped Palpatine to kill all but a few Jedi in the galaxy as well as the Separatist forces that Palpatine had been manipulating since Anakin was a child.
When Anakin finally came face to face with Obi-Wan he was almost killed in the ensuing lightsaber duel. Obi-Wan unwisely left Anakin on the brink of death instead of killing him, allowing Palpatine to find him and rebuild him as a cyborg in a specially made life-support suit. Palpatine made sure to keep the news of the birth of Anakin's children secret, ending the life of Anakin Skywalker and starting that of Darth Vader. Vader then went on to hunt down the remaining Jedi and to terrorize the galaxy for the next two decades as Palpatine's right hand man.
19 years later, Vader found out his son Luke Skywalker was alive and strong with the force just as he was. Vader spent the next several years hunting Luke to turn him to the dark side just as Palpatine had done to him. When the Empire was finally defeated, father and son met in a lightsaber duel on the second Death Star. This ended with Vader killing Palpatine to save Luke, mortally wounding himself in the process. This brought him back to the Light Side of the Force, finally bringing balance to the force.
Vader turning back to the Light side doesn't prove he was weak, it proves the Dark side was weak. It's the idea of love (which is the basis of the Light side) is stronger than hate (the Dark side).
Think about it, Anakin started of as a very honourable person and became a Jedi. He was as far to the Light side as you could be, it was only Palpatine's manipulation that brought him over. In the end, he went back to the Light because that manipulation wasn't enough to overcome who he really is deep inside.
Deep down he was still that same Anakin, Palpatine just made sure he had enough leverage to keep him locked in there. Eventually something came along that would unlock him, his son Luke.
[Edited by SuperSkyline89, 8/11/2010 5:59:37 AM]