I'm writing a novel in which the story is told from a first person pov. I don't want it to be predictable so I feel that if I kill the protagonist it will give it a sense of difference since they usually don't die and the readers know it. How can I do so?
There is a old idea that in the moment of one's death, their whole life flashes before their eyes. Perhaps your entire story, up to the point where the narrator dies, is just their giving voice to that final moment; reviewing the facts of her life on her way to face Judgement. This approach will grant your narrator hidden fore knowledge which can justify any foreshadowing which you work into your prose.
Killing your pov character midway through a story is a powerful tool, but it leaves your readers without a guide for the remainder of your tale. You can avoid this issue by splitting the pov between two characters earlier in the story. If you shift between two narrators repeatedly during the first few chapters, you can still use the survivor to narrate the remaining pages.
Finally, be careful with how you build your disposable narrator. If the soon-to-be-deceased is, in any way, a sympathetic character, then their death may wound your readers emotionally and will definitely cost you their trust. We may all enjoy a good George R.Martin novel, but we don't trust him. We have learned not to get too attached to any of his characters, because they usually don't last very long.
There are several examples that come to mind, where this technique was used effectively:
Michael Crichton - Jurassic Park - several characters (not all necessarily protagonists), are killed, and Crichton has them narrate through their own death. Definitely worth reading, I found his techniques very memorable.
In some popular movies, in different forms, offhand the following come to mind:
the recent movie "Joy" - I don't want to spoil it for anyone - but the movie is narrated by a character who dies somewhere during the movie (the character narrates before, during and after death)
some movies that are Quentin Tarantino-esq, where the character dies in an early scene - but then the rest of the movie is out of chronological order - so the main character can narrate their own death.
other movies. the main character is narrating most of the movie, and at one point, the main character is killed, and continues narrating - so you realize the entire story is being told "past tense". American Beauty is one popular example.
Also, Jesus Christ Superstar comes to mind: Judas, who is essentially the main character and (oddly) spun as both protagonist and antagonist - his death is portrayed from the first-person POV (and while singing, too! : )
I wrote a story in first person in which the narrator died. I placed the death at near the end and switched to a third person POV to give the final scene that explained the affect of the narrator's death.
I found this allowed the reader to feel like they where still in the narrator's mind, floating outside what was happening, not allowing the reader to attach themselves to someone else.
This has been done in John Grisham's The Testament. Read Chapter One which is in first-person at: John Grisham's The Testament on Amazon.
The rest of the book is what happens after the rich old man kills himself. It is possible, but it is quite odd. It's really more of a prologue that sets up the story so you could do it that way.
If the main character is eventually the enemy in the book (because she's hurting people) and her death would be a good thing for the people around her, then the end of the story would be relief on her part as she's dying or killing herself, because then she can't hurt anyone else. But the main story ends when she dies.
Your only real issue to address is what to do for the denouement, the wrap-up after her death to show that she succeeded in stopping herself from causing others more pain. Whether you change POV or have her ghost/spirit/etc. view others' reactions is up to you.
The narrator is not a person. The narrator is a function of the written text.
Think of all those first person narratives. Do they all presume that the protagonist actually sat down and wrote that text? And how would that work when the text is about the future, or a past before writing had been invented, or the protagonist does not know the language the text is written in? Some novels create a fiction of the protagonist writing their memoirs or telling their tale to the author or the author translating a found text, but even then most readers do not actually believe that.
Since the telling of the tale is not necessarily part of the narrative, you don't necessarily have to make it plausible. Just as Star Trek does not explain how we can know of what the crew of the Enterprise did in the future, you don't have to explain how a first person narrator can keep on telling their tale after death. Some readers will explain this to themselves by thinking of religious concepts of an afterlife, or superstitions of ghosts, but mostly they will not much worry about how this is possible, if you don't rub it in their face ("Hey, look, I'm still here and talking to you after my death.") and break their make-believe. Just continue as if nothing had happened (to the narrator) – which it hasn't.
John raised his gun and shot me in the face. I fell to the floor, dead. John looked at my dead body with satisfaction, then turned and walked away. Finally, he was free of me.
If that was the end of the novel, that would be perfectly fine and not irritate anyone. Switching to third person for a few sentences is completely unnecessary.
If you have a whole chapter coming after the death, switching narrators is the standard solution. But I think that you don't actually need to do that. Remaining in first person would be original and intriguing: something that readers can, but don't have to, wonder about. To me, it would add to the reading experience.