Puzzled about Charlize Theron's character in Young Adult: sick, psychotic, or just sad?

Screenwriter Diablo Cody made her rep with Juno, where she created a teenage girl who thought and behaved like a world-weary 35-year-old bitch and now, in "Young Adult," she (and dir. Jason Reitman) have built a movie around a world-weary, 35-year-old (37, actually) bitch who thinks and behaves like a teenage girl. The movie did hold my interest, but for at least the first half of the movie I kept thinking that Charlize Theron, though excellent as an actor, was far too beautiful for the part: woman who's ghost-writing a series of YA novels and gets tired of her work (oh, the woes and tribulations of being a professional writer!, or screenwriter?) and inexplicably and suddenly takes off from her dreary Minneapolis apartment to go back to the small Minn. town where she grew up - OK, the vary basic premise/starting point for many, many movies and books - but the twist here is that she's determined to win back her h.s. boyfriend, a new dad/recent first-time father. Gradually, we come to see how terribly sick and crazy Theron's Maevis is - an alcoholic and cruel, destructive, nasty person. I began to accept that in her time of trouble she'd go back to her hometown, to relive the glories of her youth when she was the town glamor queen, and where people still believe that she's a big-city success. But would she truly be so interested in winning back her totally conventional h.s. boyfriend? No - but she might be interested in destroying his life, if she's truly insane. The movie pushes us to the edge of that cliff - but doesn't go over the line, so I'm left feeling puzzled about Theron's character. Is she nuts or needy or nasty? Sick or psycho or sorrowful? Movie doesn't clarify. What it does do, unfortunately, is largely adopt the POV of Theron's Maevis: though her life isn't so great, we are also meant to see that the small town (named Mercury) is a miserable place inhabited by contemptible people: her parents who refuse to recognize her illness, the thugs who bear a young man nearly to death because they think he's gay, and others. A movie that will hold your attention but that leaves me feeling troubled by its uncertain sense of character and by its haughty disdain for the conventional lives of others.