[found on writersrelief.com; by Writers Relief Staff]

“If you’re considering killing off your main character, keep the following tips in mind:

1. Be somewhat realistic. It may be hard to swallow if your main character survives what no one should be able to. When a jumbo jet crashes in the desert but your hero, Jack, walks away unscathed thanks to his skill with a nail file and a soda can, you can practically hear your readers groan.
2. Plot problems. Don’t kill the protagonist if you are having problems with the storyline and simply don’t know what to do next: The heroine finds herself between an enraged grizzly and a cliff—if you can’t figure out a plausible way to extricate her, this shouldn’t be the only reason to kill her off.
3. Beware morality statements. Perhaps your main character’s death is a natural consequence of his fatal flaw. He is a functioning alcoholic and sometimes drinks and drives. Be very careful not to make this into a morality statement by waving it over your readers’ heads: This is what happens to drunk drivers! You want the story to be powerful, not your personal statement on drunk driving.
4. Don’t kill the MC off in a trivial or anticlimactic way. In other words, unless it’s tied to the theme or plot in some significant way, Hattie Heroine should not die from an infected paper cut. If we’ve invested in her character, we need some tension building up to her death.
5. Avoid resurrections. Please don’t be tempted to miraculously bring a main character back to life unless it’s an integral part of your plot or theme (like a medical thriller centered around a miraculous new drug that reverses death). What? It was actually Hattie Heroine’s twin sister who died of infection? Like an ending where the MC wakes up and realizes everything was just a dream, a miraculous resurrection can be a little cheesy—or an easy out.”

[found on http://writersrelief.com/blog/2013/11/main-characters-how-to-kill-your-protagonist-without-killing-your-fanbase]