Well, this is prolly an EU thing, but I read that Plo Koon can use lightning, and I heard that lightning was originally a light-side power. Maybe it's like the grip after all, just depends on how much 'umph' you put in it. Sidious only used a bit to torement luke, so if a jedi knew how to use it, he could use it to stun an enemy into submission rather than kill. I guess a dark sider once used it to kill and then everybody declares it a bad power.