Well, according to The Ausiello Report on the TV Guide webpage, the producers had always planned to kill Ana Lucia and Libby:
Carlton: Then she got this DUI and Damon and I looked at each other and we were like, "Oh, great. Everybody is going to think we're killing her off because she got a DUI." But there really wasn't anything we could do about that. I mean, the story was set. We had made plans. With Lost, we think it out well ahead of when we actually shoot it. We thought about altering our plan, but [this] was in fact what was best for the show. The fact that she got a DUI would come and go and what would live on would be the show, and our plan was still the best plan for the character. Ironically, we actually thought about changing it the other way around once [the DUI] happened, but it really was the best story. We wanted to tell the best story.
Damon: And obviously that decision was further mitigated by the fact that Libby is shot at the same time as Ana Lucia. Basically then we said, "Oh, s--t. Both Michelle and Cynthia were busted the same night for DUI and we've got this story point coming up where they essentially both get shot at the same time, so it's going to look like this is the Lost producers attempt to say, 'Don't drive drunk!'" But as Carlton says, all it created in us was [the thought that], "Maybe we shouldn't do the plan now, 'cause people are going to think this is a reaction to [the DUIs], as opposed to this [plan] existing prior to that event."
But, you know, I don't buy it.