Here’s the reality where abrupt and unhappy endings are concerned: you have to do it right or people will hate it.
Stories are supposed to have some level of resolution. Whether that means tying up all the loose threads or simply taking a look around at the fallout–that depends on the story. But you can’t just pull the plug after the climax and walk away. The climax is an action scene that needs a reaction scene. You at least need to show the protagonist looking around as the dust of the climax settles around them, and throw in some measure of possibility for what might happen next.
And unhappy endings should only be done in stories where a happy ending was never expected or promised–stories that are dark and bleak from the beginning, even if there were moments of fleeting hope throughout. But if you set your reader off on a course where they expect a happy ending–or at least a hopeful one–and then you dish out an unhappy ending, your reader will feel unsatisfied by the ending.
Abrupt endings are even trickier. They should only be done when there’s been at least some level of resolution after the climax. You can’t have the knight plunge his sword into the heart of the malevolent dragon that was plaguing the village and end it there. That’s just lazy. A lot of writers think the shock delivered by an abrupt ending will delight the reader, but most of the time that isn’t true. The reader will only enjoy the shock if there’s value in not knowing exactly what happens next. In other words, everything else has to be tied up more or less. You can leave the reader wondering exactly what happens next, but you shouldn’t leave them with a thousand unanswered questions.
So, ultimately, you have to ask yourself why you want your story to end in this abrupt place. Does it serve a purpose beyond just shocking the reader? Do you know how the story ends even if you’re not going to share it with the reader? (If not, are you just choosing to end the story here to avoid coming up with an ending?) Have you left the reader with a few possibilities of what happens next so they can at least decide for themselves?
Consider all of that and you’ll be on the right track! 🙂