Jane's motive
I get the impression from the other posts and comments on this board that you people don't seem to think this movie establishes an adequate motive for Jane. This is my response to that complaint.
There are clues in the movie that she does in fact HAVE a motive. In the very first scene, before the opening credits roll, Jane attacks Jessie's sister in the woods. As she is attacking, one of the things Jane yells is "How could you do this to me?" In other words, Jessie's sister has done something to Jane in the past that Jane is mad about. That Jane brings this up during the attack signals (to me at least) that revenge for whatever it was that Jessie's sister did is what motivates Jane. Now--
That point was something I noticed during the first time I viewed the movie. I remember watching the remainder of the movie with the question in mind, "When will we find out what it was that Jessie's sister did to Jane?" And of course, we never do find out. We are put several times in the position of almost-finding-out, but the movie (INTENTIONALLY, it must be noted) never actually tells us. What this means, I would venture to say, is that we're not supposed to know. What it means is that THE DETAILS OF JANE'S MOTIVE, ALTHOUGH SHE DOES INDEED HAVE A MOTIVE, ARE UNIMPORTANT TO THE PLOT OF THE FILM. The filmmakers were aware of this fact and intentionally played with it. This is why, time after time, when people ask Jane why she's doing all of this, she makes fun of them. "Why? Why? Everyone wants to know why!" That sort of thing. It happens several times.
Yes, Jane is crazy. Yes, that is what ENABLES her to murder. No, that is not what CAUSES her to murder. What causes her to murder is what Jessie's sister did to Jane in the past, the details of which are never revealed. To reiterate, the reason the details are not revealed is because they do not matter. That Jane is crazy and that she has a vendetta of some sort against Jessie et al are sufficient in getting the plot rolling.
(For those familiar with Hitchcock, there are parallels to be made between Jane's motive and the maguffins in his films; they are there to set the plot into motion but beyond that are unimportant. What matters in stories such as these is not what causes the ball to start rolling but what happens in the time that it is rolling.)