Not so much that he isn't immersed in the Dark Side, Zell, but that he isn't actively using any Dark Side powers. Before the Jedi Council confronts Palpatine, he looks relatively normal. It's after he kills them all (using Dark Side forces, no doubt) that he begins to become disfigured.
We see a similar degredation of Anakin as he turns to the Dark Side.
I think the decay has to do with USING the Dark Side, not neccessarily simply being a Jedi. Following the Sith religion isn't the catalyst, it's the 'evil' powers that you use. Presumably, to use them saps body energy, or something...
[size=1]"So the woman asked me what I wanted on the sandwich and I said I do not care it is for a duck, and she was like oh then it's free. I was not aware that ducks eat for free at Subway. It's like give me a chicken fajita sub, but don't worry about ringing it up, it is for a duck.”
-Mitch Hedberg
[/size]