I need some help

Everyone dying at the end of a story is always disappointing, even though sometimes that's what happens in life, depending on how tragic the situation is. I personally wouldn't suggest killing off all of your characters, since it may make the story seem a bit rushed as well. Just my personal opinion, but it's your choice. ^^

kill off some but not all and make them remember the tragedy of losing the other characters, then tell about how they live on. or something idk.

Killing off all of your characters is usually not a good idea. Not only is it generally very depressing, but it makes it seem like you Nawawala interest in what you were writing. Personally, I like LocalArtistist's idea.

I wouldn't kill them all.

I say you make it look like everyone died but turns out one person survives and has to be the saviour. Do a epilogue then start your new book! Put the saviour part in the epilogue or don't do an epilogue and put 'everyone died but one' or something like then end it as a cliffhanger! Is this a really book or a fanfiction or a fanfic?