and just to add on to my last post, heres a quote I just found from an old interview
Damon Lindelof: And all of this was obviously, first and foremost, the idea that Libby's got this mysterious backstory, of which we've only given you the tip of the iceberg. We know she's spent some time in the mental institution with Hurley, and the idea of killing her before she had an opportunity to explain how she got there... we have a master plan for how we're going to tell that story, but it's all posthumous. You'll start to learn Libby's moves through flashbacks over the course of the next season. So we're not done with Cynthia, but Libby is dead.