Xjyuk

I have a five book series and I have plotted them all out. The main bad guy wins in the end. My problem with this is my girlfriend keeps telling me that bad guys winning will make readers upset that they invested in the heroes only for them to die or lose. So now I am second guessing the entire series.

Do you guys think that it would be ok for a bad guy to win in the end?

It is perfectly fine for your story to end with the "bad guy" winning. Consider for example George Orwell's 1984:

He loved Big Brother

Complete and utter defeat. 1984 is one of last century's masterpieces.

@Wetcircuit mentions tragedy in a comment, for good reason. Tragedy does not necessarily imply that the "bad guys" win, but it does imply the "good guys" lose, or at best earn a Pyrrhic victory. Consider Antigone or Hamlet, or For Whom the Bell Tolls. In fact, tragedy is often considered a "higher", more "literary" form.

Yes, your readers are going to be upset when your characters die or lose. At least, hopefully they will have come to care about your characters, so their death would sadden them. But that is not a bad thing. One feels sorrow when one finishes For Whom the Bell Tolls, but does one go "what a bad, disappointing book?" Never! On the contrary - one is profoundly touched by that sorrow, one appreciates more the fleeting beauty of life through it. @Amadeus apparently looks for entertainment in the books he reads. Me - I look for art. I look for that which would touch me, and take me out of my comfort zone, and make me think. Formulaic "good guys defeat bad guys, then live happily ever after" bores me out of my mind.

Now, there is a question of what you're trying to say with your story. Why does your "bad guy" win? What does it all imply? If all your story suggests is futility, for example, then your readers might well be disappointed. But if your story does have something else in it, like any of the examples I've mentioned above, or countless others, then go ahead.

You can do it. But the expectation has to be set that this is possible, and it should be written like a tragedy. The market for such a book may be small, but it isn't nonexistent.

Check out the grimdark genre (third law series is an example). People do buy into it and even like it. But it's unlikely to sell as wide as something that has a feel good ending.

Or, go see the musical Hamilton.

As a writer you'll have to be excellent to even have a chance. And putting "the tragedy of" in your title might not be a bad idea.

No, I don't think it would be okay for a bad guy to win in the end.

Readers don't like it. They read for fantasy fulfillment. Happy endings outsell unhappy endings ten to one; publishers and studios don't like unhappy endings. They want something positive in the end.

Especially from a writer that has no following; if you were already a best-selling author or script-writer they might trust you and publish it anyway, but not if you are starting out.

In a series you can have a mixed ending; basically a draw. The hero doesn't win, but doesn't lose. But even that might not be satisfying.

If you are unpublished, you probably should not be writing a series, unless you intend to write all of it before trying to sell it. Publishers do not want to publish book one with an ambiguous ending if there is no guarantee you will actually finish the rest of the series. And if you are a beginner, they don't want to buy three or five books at once. And if your series has an unhappy ending, they don't want to buy any of it.

I suggest you write a book, even a somewhat long book, that stands on its own, with a reasonably happy ending in which the hero prevails, perhaps at a cost but prevails. The villain is defeated, perhaps escaping with their life and bound to return, but defeated.

The problem here is psychological. Reading fiction is escapism. What are readers trying to escape? The real world, where the bad guys win pretty much all the time! In real life, crime pays. People get away with rape and murder and abuse of others. Drug kingpins, dictators, corrupt politicians destroy innocent lives and live high on the hog without a single regret.

The real world is what we are trying to get away from. We want you to make your story and setting believable, and the dangers feel real, but in the end we don't want the realism of the hero chickening out, or the bad guys prevailing and continuing to create pain, misery and hopelessness. In the end, we want the wish fulfillment fantasy that the good will prevail and the nightmare will end.