I think the producers/writers killed off Charlie, coz' Evangeline Lily is becoming a "bigger" star and Dominic Monaghan, her boyfriend still remains as big (or little) as a hobbit.

You can kinda tell she's not going to be with him for much longer as her fame grows. A bit shallow thinking, true. But with her on scene romp with Jack / Matthew Fox and Sawyer / Josh Holloway (though both are married), her popularity and (sex) appeal is rising.

So, kill of the hobbit and keep the femme fatale.

If they wanted to keep him around longer, there are many ways to do it. Locke can get shot and still be alive. Ben can have tumor and still alive.

The "300" actor guy gets bitten by a spider and he gets buried alive with his gf.