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