FAQ for
Casshern (2004) More at IMDbPro »

There is no undisputable clear-cut answer to this question, so here's an interpretation:

What does a lightning bolt do ? It can be regarded as a form of divine punishment, but also as life-giving energy. And in the case of Casshern: both.

Where did it come from ? Casshern is the one who's sent the lightning bolt. He is a sort of guardian spirit (in charge of protecting the people from the "black-and-white village"). He can be seen in the first shot of the film with a lightning bolt in his hand (that's a pretty big clue).

As to "why" he did it: He was probably trying to teach Dr Azuma a lesson (a lesson he will never learn). After all, Dr Azuma commited the ultimate sin: he was never able to create those so-called neo-cells (which were meant to facilitate transplants and clone organs). What he created was a liquid that could regenerate living things. Note: As I understand it, in japanese culture immortality is not a good thing. Life is a gift, and so is death. You want to prolong life, that's okay. But cheating death isn't.

Let's see if this makes sense, now: 1. Poor defenceless people get slaughtered and chopped to bits. 2. Body parts are sent to Dr Azuma's lab. 3. Angry guardian spirit Casshern throws a lightning bolt at Dr Azuma's lab to revive them and give them a second chance (and teach Dr Azuma / Humanity a lesson they won't forget).

Sidenote 1: It is quite clear that it is also Casshern who offers a shiny castle filled to the brim with killer robots to the mutants (it probably is his answer to the mutant who cries out to him when they are crawling in the snow).

Sidenote 2: It is also Casshern who chooses to destroy the lightning bolt and return Tetsuya to his home. As to "why": Tetsuya has decided to call himself Casshern. This way, he has become the mutants' new protector. Casshern (the guardian spirit) is no longer needed.

This is just my interpretation. Don't quote me on it, maybe I got it all wrong.