As a thought, according to the Episode III Novel, Mace Windu's Lightsaber Form, called Vaapad (Form VII), the user of the form draws from deep emotions, which takes them very good to the dark side.
Mace Windu was a MASTER of Vaapad, meaning he was so good at it, that the Force Lightning (a Dark Side ability) practically went through him, not harming him at all, at least while he was weilding his lightsaber like a lightning rod, and extremely focused. Mace was essentially BEATING Palpatine in their duel, but was only defeated because of Anakin's interference