I nominate Anakin/Vader, even though he is a bit cliche. His entire motivation for going to the dark side is not lust for power but fear of loss. Everything he's ever called his own was stripped away from him, so when given a chance to gain power to make it stop, he siezes it. Of course his fear and consequential need to be in control result in him destroying what he loves, and is left with nothing, not even his own life, to call his own. Literally now owing his very existence to the Empire, he then tries to prevent its descent into chaos the only way he knows how; to control it with an iron fist to prevent all change, not just bad change. To this end, he secretly wants to destroy the Emperor so he can be the one to "save" the galaxy. His final closure lay in his son, who proved to him that even lost causes could be salvaged, and Padme's death was not the death of hope.
At least, that's how I look at it. While I have a few other favorites, Vader tops the list because of why he did what he did.