To answer these questions: When a Jedi is seduced to the Dark Side, the destructive energy of the Dark Side begins to take a toll on the Dark Jedi's body, (ie, Emperor, who had to keep switching bodies to stay alive). Kyle's body was affected in the same way when he went to the dark side, I assume after you leave it the affects subside or the power of the light side restores your body (either through himself, or Mara or Luke). That is why Dark Jedi can come back to the light (ie Kip Durron, Kam Solusar)
"ITS JUST A DUAL LIGHTSABER SKIN YOU NEWBIES!!!"- me, Jedi Knight on the zone.