For me, it was a culmination of two things:
#1: I had two characters that were very similar. Many of my reviewers commented that characters A and B had very distinct voices, but C and D sounded the same. They couldn't tell who was saying what without the names, and neither one of them stood out strongly. Several chapters in to the novel, one reviewer said, "We finally got to see a little bit of C's personality when she and B had a heart to heart talk. Up until now, she's been kind of a shadow character. This was nice to see."
Nobody likes hearing that they have a character with no voice, and I was particularly attached to this character. At one point, she was THE main character. But slowly character A became larger and larger, taking over the role of main character. As A's character developed, C became smaller and smaller. When enough people commented that C and D were too similar, I made the difficult decision to get rid of one.
#2: The book needed something to jump-start it. My readers started chapters 1 and 2 thinking they were reading a 'slice-of-life' novel, when in actuality it is a psychological thriller. Right away, I needed to let my readers know what was at stake. I decided to start the book with a murder scene.
But who to murder? Well, that was pretty easy, since I'd already decided that character C needed to go. I had two choices: simply have the girls be a trio, or have them start as a quartet and kill of C. I decided to kill her off.
I knew that I would instantly accomplish two things: alienate some readers who couldn't handle the murder of a teenager, and reel in everyone else when they realized this is a high-stakes novel. Even main characters can die.
It was hard to take away C's future. She had plans in the novel, and the moment I killed her, I took all of it away. There were lives she would no longer touch, people who would no longer love her. I mourned for one boy in particular, who was supposed to find his soul mate in C, and instead he ends up alone.
But it was also far too easy. Almost every part she played in the novel, I could assign to character D. That's how I knew I'd made the right decision. My critiquers are usually spot on.
Have you ever had to make a life or death decision with a character? How did you deal with it, especially if you decided to kill the character? And if you're not a writer, what color socks are you wearing?