Do you kill the spider to save the butterfly?
In the Trigun episode entitled Rem Saverem, the audience is treated to a revealing look at the past of Vash and Knives, twins who have grown up to become the hero and villain of the series.
Their origin is not clear, but they seem to be a race of alien beings who were adopted by the crew on a human spaceship called "Project SEED". The idea behind this project was that humanity was fleeing an uninhabitable Earth and trying to find a safe haven for their kind to prosper.
On the ship, many people sleep in a state of stasis while the crew charged with keeping them alive consists of only a few people. Rem Saverem is one of those people, a naïve, joyful woman who's always teaching the childlike beings Vash and Knives about her hopes and dreams for a brighter future for humanity.
These beings seem to grow to maturity faster than humans and also possess higher intelligence than humans. This makes one member of the crew jealous and suspicious of them. Certain events on the ship unfold creating a bitter Knives who seeks the destruction of humanity for the good of the Universe (because he believes that since humans killed the healthy ecosystem of Earth they are a kind of pathogenic organism that must not spread across a healthy universe), and a Vash who continues to vehemently profess Rem's ideals of love, peace, and hope for a better future.
A lot of the dramatic tension of the show revolves around the philosophical differences between Vash and Knives. Knives wants to create a future where humans are dead and only him and his brother remain. Vash wants to create a future where humans live in peace, without violence. To achieve his aims, he refuses to kill. He will go to great lengths to protect human life, even in the cases of "bad" people. He proves himself willing on many occasions to endure suffering on behalf of humanity.
Why does he do this? Well, it mostly boils down to an argument Vash and Knives had as children aboard the SEED ship. The ship contained a kind of holo-deck recreation of what a green meadow back on Earth would have looked like, complete with plant and animal life. The brothers came upon a butterfly caught in a spider's web, with the spider advancing, ready to attack it's prey...
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And then Knives killed the spider.
Vash started crying, saying "I wanted to save them both, you idiot!", and they get into a fight.
But the truth is, this seems like a case of Broken Aesop: because what Knives says then, that you can't save them both, because then the spider would just die of starvation, is totally correct. It is naïve as he said to think you can save them both.
However, if Knives is right about the spider, does that make him right about humans? If you had heard about an alien life form that had been responsible for the destruction of their own planet, do you think you'd react to them the way Knives or Vash did? I would have to say Vash, because I value the life of all sentient beings. However, Knives poses an interesting question: if humans represent "the spider", does it mean that in order to save our prey, the ecosystems, we have to be killed? The truth is, unlike a mindless animal like a spider, we can choose to live a different life. I think that's what Vash is trying to show people with his path of relentless self-sacrifice. Perhaps in nature, predator and prey should be left alone because they both complete the balance of nature.
So, this relates to the central philosophical question of the whole show: should we be allowed to kill bad people?
Other questions that are raised: Is humanity, when you really look at all the harm we do to other species and each other, worth protecting?
If humans are so flawed, why do we still have the will to survive?
Is killing justified if it's to save something innocent and beautiful from destruction? Do you kill the spider to save the butterfly?