Killing Off a Main Character?

My current project, Heir Apparent, is my first tale to reuse a character from a previous novel.

For that character to work properly in the story, I need an extensive set-up required.  He is, after all, a problem solver, not a direct participant in the crimes and misdeeds.

That left me with the question of how to move the story along prior to his arrival.  No problem.  I simply created a character -- a cop, in this case -- through whose eyes much of the early part of the story could be revealed.  Of course, there is also the antagonist.  But the antagonist is disguised, because I don't want the reader to know his/her identity until later in the novel.

So, I've been merrily writing along, using my cop to explore the first two murders.  I didn't want the cop to be boring, however, so I make her a Sherlock Holmes type, who is really good at seeing threads of evidence and weaving them together.  She's a former beat cop, whom the chief of police recognized as a diamond in the rough and decided to mentor.  Now she's on the homicide squad, and many of the senior guys there think it's only because of her friendship with the Chief.  She's a hard-working single parent who also cares for her aging mother.  And she's funny.  And she's going to fall for the protagonist, when he evantually shows up.

The only problem is, in the original design of the novel -- she gets killed.  Unfortuantely, I quite like the character, and now am having trouble with the murder idea.

Her death is true to the original design of the story, and consistent with my philosophy that these somewhat dark corporate thrillers absolutely can not end with a pretty pony party (my apologies to Lemony Snickett).

On the other hand, her character is fun, and might also be good for a future story.  And there's that emotional attachment I've developed...

Oh, what to do?  What to do?