Ok, there has been a BIG talk about this in the Outcast forums. Here is the position
1. In Single player mode, Kyle's moral position is tenuous; he can go good or bad.
2. In multi-player mode, powers will be split into Light and Dark. While this is not an accurate reprensentation of how the Force works, Raven felt it worked better game-play wise to have it split (and asked for permission from Lucasarts to do so). So in multi-player, only the Dark Side guys can use Lightning.
I think you'll find piddling around with Lightning in single player is likely to put you down the evil path, yerss.
Besids, if any power in Star Wars could be said to be Dark Side only, it's the Ligntnming.
BTW, while you choose your powers in multi-player you don;t in single player. No-one is 100% sure how the advancment works in singe-player but it is something to do with Kyle getting better with the powers you use more often (sabre skill is counted as a Force power). Like I say, we don't know the details.