-Well, if you follow the story I think you'll find the Emperor probably did it - as the medical droid tellingly said, "She appears to have lost the will to live", and given the Emperors need to undermine Vader's sense of "Good", he probably did that to her and then told Vader he killed her, with a small smile on his face when Vader goes a tad more evil. It removed the one "good" person remaining in his life whom he cared for and made him all the more scarred and evil. - AMK