Genres are used to inform potential readers that your story will interest them. If you put false genres or tags, they'll get upset when they realize you lied. But the more you add, the more people will look at your story. So put as many as possible without lying or misleading the audience.
And yes, include tags for things that aren't in the story yet. It's better for a new reader to get upset about why the tag isn't present than for a long time reader to drop the book because the plot suddenly swerved in a direction they weren't expecting.
If it only happens in one chapter, you might be better off using another tag since you can only have 25 tags. If you don't have 25 already though, go ahead. Some tags exist that can only happen once by nature. If it's an unpleasant tag though, you should consider using it or at least putting a content warning on that chapter. To avoid surprising your readers, as I mentioned before. If you feel guilty about it, you could always dedicate more thought to the tag.