As the big bad writer, you’re the ultimate authority on what you want to do with your story.
Do you want to kill off everyone in the end? In that case, go for it. Eliminate them all.
Do you want to have the couple that seemed so destined for eternal bliss break up? You’re the boss.
Do you want to have the little sister who’s acting as a nurse in the final fight scene get blown up by friendly fire for no good reason other than the fact that you’re an intensely unhappy person who should probably be checked into a psych ward?
Well, Suzanne Collins, I suggest seeing a professional about that. But if that’s what your twisted writer brain desires, then so it shall be written. So it shall be done.
Just understand that your readers might hate you after the fact. And you might be killing your writing career in the process.
Since I don’t know you, I can’t say for sure that it isn’t your goal to be despised by readers. Perhaps profits displease you, and your only passion in life is to overwhelmingly disappoint everyone.
If that doesn’t quite sound right though, you might want to consider your readers while writing your final pages. (Actually, you might want to consider your readers the whole way through.)
Infuriating your readers at the end isn’t often advisable.
It’s your story, so you can end it however you want to end it. Just understand that there will be consequences, whether good or bad, according to that end.
You’re asking for trouble if you try to write an ending that doesn’t fit your chosen genre. Certain literary categories lend better to certain endings, and it’s important to know which are which. If you try out a trick conclusion on the wrong kind of story, your publisher and/or readers will not be happy with you. And they will let you know somehow, someway.
If you’re self-published, that “somehow, someway” will be through bad reviews. That and people vowing to never read you again.
If you’re a candidate for a traditional publishing contract, you’re safe from that fate. But only because your publisher-to-be will tell you to rewrite the whole ending.
Being a business and all, they want to make money. Even if you don’t.
Going back to that whole "killing everyone" bit from the beginning... When writing horror? Sure. If that’s what you want, then there’s nobody stopping in any way, shape or form.
Not only are you the writer and therefore the boss… but your readers understand that the genre they’ve selected means anything nasty is up for grabs. (And if they don’t, well, that’s on them.)
For that matter, even the ending of The Mocking Jay, where Katniss’ little sister gets blown up? That might not have been out of order. Although I’ll never read another Suzanne Collins’ book ever again as a result.
And I maintain she has some serious issues.
Even so, dystopian novels aren’t exactly advertised as being rays of sunshine. So I guess she was within her genre-specific rights to do what she did.
However, if you’re writing a romance novel, understand that you’re playing with fire – and not the sexy, steamy kind – if it doesn’t end happily. Not necessarily happily ever after, mind you, but happily nonetheless.
The same goes for chicklit. And, except in special cases, a mystery had better see the case cracked by the end of the book.
Basically, keep this mantra in mind, and you’ll do just fine in writing a "killer" ending. Happy readers means happy writers… who have some cash to burn.