M.Night Shyamalan is Genius.
His idea is marvelous. The message the movie trying to pass to us is, human not very care about the nature until some day the plant also feel threat from the human being. That is why the plant is releasing some kind of chemical in the air to kill human.

In my opinion, the movie has more literature element in it.

Elliot represent human kind. And the old woman in the last scene represent nature, where she always felt threatened by Elliot. She said Elliot is trying to kill her. Elliot try to steal something from her.

Anyway, this is my conslusion.