Accident or Suicide?

I'm really torn as to whether or not Lily's death was on purpose or an accident. I really can't believe that Lily accidentally took too much of her sleep medicine. She is much to calculated and methodical to slip up so badly. She does everything with a purpose. Throughout the book, we have identified her as manipulative and cunning. And her conversation with Selden is too ominous for her death to be an accident - "Let us always be friends. Then I shall feel safe, whatever happens" (309).
However, there is a definite change that takes place during the course of the novel. She is extremely depressed. Perhaps her depression overcame her and she wanted to take more than just a "brief bath of oblivion"(322).