Rules, No. Suggestions, Yes.

Most fiction writers will talk about their characters taking on a life of their own. Those writers will tell stories about having an idea or intention, but that their characters disagreed and took the story where they wanted it to go.

It always sounds silly and at least a little crazy. If you create your characters they do whatever you make them do. How can they control their own creator?

The reason is that the creator is receptive and responsive. If they create a story where the main character ends up killing their antagonist, the main character has to have certain characteristics. However, if you create a different main character, then that character won't have the personality traits that would have them commit callous murder like that.

Now, sure, you can force your character to kill the villain in the end, but you run into problems. It is out of character. It doesn't make sense. It destroys any reality that you have established in all of the story up to that point. If you want to make a good story, then the character traits you create will guide their actions more than your whims or desires.

There is nothing wrong with planning out the journey of your story. Figure out where you start, where you end, and important checkpoints along the way. However, realize that these are suggestions, not rules. As you are writing, you may come to realize that your characters would naturally do something different than you originally planned. If you experience this, always choose whichever is most natural. If that means revising your original path, then do so. It will make for a better story.