Given that Palpatine was monologuing when he unleashed the Force Lightning, I think it's a fair conclusion that when he started zapping Luke he was using a low-power version of Force Lightning. We saw what Force Lightning can do to a man's face, but Luke still has his fresh looks; it looks like the initial blasts are intended to immobilize and to torture but not kill, not yet; first Palpatine has to get through his own Villain. It's only after he says "And now, young Skywalker, you will die" that he powers up to killing strength which WOULD have resulted in an agonizing death for Luke, except that Vader stepped in and sacrificed himself, taking the lightning meant for Luke. Whether from destroying his life support or simply from damage, these wounds were fatal to Vader, although it still took him several minutes to actually die.

It kind of suggests that Force Lightning really isn't a particular useful tool for quickly killing people; which seems odd, as the Electric Chair used to be used for executions primarily because it could kill nearly-instantaneously. For some reason the Sith never use it this way; we've never seen anyone actually killed with Force Lightning. Even Mace Windu didn't die on the spot; he had to be thrown out a window. Is this a weakness of Force Lightning vis-a-vis real electricity? Or does it tell us something about the Sith themselves?

Respectfully,

Brian P.