The difference between god and batman

[Angrboda]

This leads to the situation where Batman (or Superman, where Lex Luthor is concerned) refuses to kill the Joker when he has the opportunity, even though he must know that the Joker will escape again. The Joker's body count over the past decades may well be in the hundreds of thousands, but the next time Batman catches up to him, he'll spare his life so as "not to become like him." It's a necessary plot device in a medium like comics.

In the legends of India, because of the common belief in reincarnation, there are tales of fated pairs, one good, one evil, who are destined to play out their respective karmic skeins over and over again, always coming back to play an essential role in the karmic path of the other. In Jaina legend, there is the story (or stories) of Kamatha, the evil one, and Vajraghosa, the saint. Many incarnations end with Kmatha bringing about the death of Vajraghosa in remembered rage from past lives, and having such action causing him to be 'sent down' in the next life due to the enormous amount of bad karma generated by the act. Vajraghosa, on the other hand, displays ultimate character and virtue in his response to each death, resulting in a massive boost of good karma, propelling him to a higher plane of existence in that next life. I'm also reminded of the Star Trek Next Generation episode in which the Sun and the Moon, personified as deities by a long dead culture, continue to pursue each other in a recurrent circle of pursuit and capture.

So this suggests to me that there may be more than 'mere plot device' afoot here, and that this may be a reflection of a more substantial, and enduring, theme or trope in myth and story. What it exactly means and what its significance is, on the other hand, I don't know. (My mind wants to impose some sort of triad upon it, to make it align with the standard mythological component of things coming in threes, but perhaps I'm reaching.)