"Wait...The main character dies?"?

When you get asked a question like that a lot, its because you didn't foreshadow properly. You've set your story up as one where no one important ever dies, you've made that promise to the reader, and then you break that promise by killing someone important. Of course they're upset.

The solution isn't to change the ending, but rather to foreshadow it better.

Allow me to use the movie Serenity as an example (warning, spoilers): one of the beloved main characters dies, and while that upset a lot of fans, you don't see too much of the rage-quit that you're describing. The reason is that Joss Whedon foreshadowed death throughout the movie. Not that character's death specifically, but we were shown that this is a world in which anyone could die.

It sounds like your story is one where no one dies, or at least not anyone we care about. So, foreshadow that your story is the kind of story where a main character can die.

Another potential problem is that your foreshadowing existed, but was wrong. That is, you may have promised via foreshadowing that the character would do something, and then you killed the character with that something left undone, thereby breaking a promise you made to the reader. In such an event, again, it's natural that they'll be upset. You need to go back to figure out what you were promising about that character.

And finally, people thinking that the death is necessary for the plot STILL means that you're doing it wrong. If a reader has to appeal to plot, that means that they are kicked out of the story and are now analyzing it on a totally different level. You've kicked them out of the story, so you need to find out what you are doing wrong and fix it.

The fact that they are appealing to the overt plot structure tells me that likely the death wasn't earned. The character didn't die because of their actions/goals, or the actions/goal of other characters, but because the plot needed them to die. The rub is, the plot isn't an acting agent. If a character dies because of plot, then that is just a form of deus ex machina. You need to go back through your story and figure out what would need to happen in order to justify that character dying. Maybe it's the logical result of their own choices (Obi Wan fighting Darth Vader, then stopping and letting himself be killed), maybe it's the result of someone else's choices (the Operative killing Shepard Book in order to eliminate the safehaven of the people he's hunting), but the key element is that it has to grow out of the story, not be forced onto it.

EDIT: To be clear, these reactions are not the reactions of people who liked your character and who are sad that the character died (the mark of a good character death), these are the reactions of readers who feel that you broke your promises and thus don't deserve their trust. Never break a reader's trust. Lie to them, sure (that's what being a fiction writer is all about, after all), but don't break their trust.

A great quality in an actor is the capability to kill off characters. It was something I fought with. I'd been writing my character for a year and then I just killed her off. It was awful and on some levels it felt as if I was killing a friend. But it's really a way to expand your knowledge and your quality as an author.

Don't worry too much about the reactions. People are bound to freak out when they enjoy a character and the character just dies. But truth is, people die, and the world where everyone lives is so vastly unrealistic that it's almost ridicolous.

Nice to see that you're taking the next step of writing.

If the story calls for it then enough said. I've never killed off a main but I have had my main raped. I felt sick and mulled over it for weeks but in the end, I wrote it (not the actual rape bit but before/after). I know that it couldn't have gone down any other way and it needed to happen for the story to progress. If I had written it any other way, it would have felt wrong. Just write it how you want. Its okay to take their opinions into consideration but in the end, it is your story. Good luck!

Mostly the people who say "OMG NO DONT KILL HIM!" Are major fangirls. People who find it interesting are writers.

As both, I find myself having conflicted feelings. Most of the time when I'm writing and kill off an important, if not main, character, two halves of me are going- "Whoa, this is gonna turn out good." While the other half is like- "RAHHH HOW COULD YOU! YOURE BREAKING EVERY (non existing) RULE!"

Just do it, if that's what you want to do!

May I suggest that if people are begging you to change the story, and are really sad that your protagonist dies, you've got a good story on your hands.