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No one needs to do anything. It is your story. There are plenty of stories without death. One option that could also allow you to go on with a series is to have it so the Antagonist is "Driven out ...
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#3: Attribution notice added
Source: https://writers.stackexchange.com/a/31048 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
No one needs to do anything. It is your story. There are plenty of stories without death. One option that could also allow you to go on with a series is to have it so the Antagonist is "Driven out of town". As the old westerns would say "This town is only big enough for one of us." You may not kill him, but you can have him run out of town. His operation thwarted, no goons left to help him. Then he comes back with more help and repeats the cycle if you wish. Or as you said, he can just die from other means. Or maybe the villian truly turns good again like Vader. There are a lot of ways to create a "happy ending". Personally, I like my violence and meaningful tragic deaths (you can thank Anime for that one). In the end a story is as interesting as you make it. A story about a guy stranded on an island who talks to a beach ball for years sounds absolutely horrid. In actuality, probably one of the better movies in the last couple decades. Not everything needs death, just a sprinkle of the right touch. EDIT: Was going to write this as a comment on my post but realized it was heading for a longer than expected explanation. I have never been a fan of overly done violence myself. The movies where they completely blow it out of proportion are just not my thing either. I don't mind mild violence though like LoTR. I have a hard time even dealing with realistic violence, but at least it is realistic. However, I like fighting. Not because I love seeing someone have their head cut off, but because I love the art of war. How can war be artistic? There is a beauty in a well executed strategy, out witting your opponent. There is a beauty in fighting in itself. Martial "Arts". Technique, training, skill, dedication. The Samurai code of conduct also known as the [Bushido code](https://bushidocodeyear9ses.weebly.com/bushido-code-7-virtues.html). Like many say there is a lost art in being a gentlemen, there too is a lost art to being a warrior. That is why I personally love and am writing a story with war and fighting. Will some hate me for it? You might. But that's okay. This is my story, and something I am passionate about. I hope that you don't let the masses turn you away from what you are passionate about. Hold close your ideas, Socrates was killed for his beliefs, but ends up being a cornerstone in modern philosophy. Do what you think is right! I am nothing but a samurai in a rice field, at peace but always waiting and always watching.