Darth Vader, Dark Lord of the SIth, wasn't always evil. He was born Anakin Skywalker. After a chance meeting Qui-Gon Jinn it was presumed he was the one spoken of by an ancient Jedi prophesy. As a pupil of Obi-Wan Kenobi he trained as a Jedi and became very powerful in the Force. His devotion to his wife Padme, who he married secretly, was his downfall. Darth Sidious convinced Anakin that the only way to prevent her death was through the Dark Side of the Force. Anakin was forced to choose between saving The Republic or saving Padme and in the end he failed both.
When Vader challenged Obi-Wan he was defeated and left for dead. Darth Sidious kept Vader alive by incasing him in a mostly cybernetic body. Vader dedicated himself to restoring peace to the galaxy by enforcing the evil will of the Emperor no matter what the cost. In the end Vader was forced to choose between saving the life of his son, Luke Skywalker, or serving the Emperor. Vader chose to kill Darth Sidious thereby bringing balance back to the force and fulfilling the Jedi Prophecy.