To Kill OR Not To Kill
Question: Is it okay to kill one of your main characters if you have more than one main character?
You can kill a main character if he/she is the only main character. It's a common practice in tragedy. You can also have a main character sacrifice him/herself for the good of others.
This is a little easier if you are writing in third person rather than first, since anything that follows the main character's death must be written from someone else's point of view.
On the other hand, it's hard to have a happy ending if your main character dies, since the Judgement will usually be Bad. (Judgement, in Dramatica, answers the question of whether the main character ends up happier, better off, or more content at the end of the story.) Most likely a story where the main character dies will either be tragedy or comi-tragedy.
Incidentally, when I use the term "main character," I'm using a technical definition. The main character is ...
* the primary point-of-view character.
* the character whose decision at the climax determines if the story goal will be achieved.
* the character the reader has the most empathy for.
If you simply want to kill one of the other major characters, you have even less to worry about. Just make sure the death serves the interests of the story.