I actually have a theory on this. This is a very pivotal scene in the movie, even in the context of the whole Star Wars universe. Anakin is given a choice between good and evil, but whenever the dark side is in play the outcome is always a little shady (no pun intended). Mace Windu for some reason changes his mind from bringing the future emperor in to killing him, which is when Anakin walks in and says:
"I am going to end this once and for all! - Mace "You can't, he must stand trial!" -Anakin
And then later on:
"He's too powerful to be left alive! -Windu "It's not the Jedi way" -Anakin
This is only after force lightning is applied. Even if the lightning had not struck Mace he did need to use Vaapaad which some consider a dangerous and possibly dark technique to deflect the force lighting back at Palpatine. I am of the belief that the usage of dark powers affects ones beliefs, after the lightning battle Windu had lost all ability to reason and could think only of killing the Emperor for the greater Good. Palpatine's lightning was being redirected back at him and was actually being used defensively.
It is possible that the use of the lightning and the effect on the person being hit is such a strong force ability that it will actually warp the wills of others around them to cause them to feel pity towards the one being attacked.
I am saying it is like a case of deja-vu. Lightning striking Palpatine caused Anakin to betray the Jedi, striking Luke, Anakin betrayed the Emperor. Remember because of Anakin's high midichlorian count he is very tied in with the ways of the force.
And in the case of Vader saving Luke, this was probably more so given the fact that it was also his son.