I don't think Leia even needs the greater good as motivation. Leia is very passionate and aggressive, she's absolutely full of anger, fear and pain and more importantly she doesn't show much or any control over it. In an emotional confrontation, the dark side would be with her in an instant and she'd use it without even realizing it, it would happen very quickly, making her mental and emotional toughness mostly irrelevant. How can she resist if she doesn't even know what the dark side is? To her it's just emotions. Why would she want to resist if it helps her fight back? She'd probably like using the dark side to smack Vader around a bit. In her mind she wouldn't be turning evil (that would come later down the road) she'd just be fighting back against evil. Luke realizes the folly of using his rage to fight the dark side because he's been taught the Jedi way, Leia hasn't. Really, Vader and Palpatine wouldn't even need to do much to get Leia to use the dark side, they'd just need to trigger an emotional response. They don't even need to mention the dark side or anger or hate, that would come all on its own because Leia would bring it with her.