“I always wanted to be in the movies,” says Aileen Wuornos, the hard-bitten, homeless soul who turns tricks on Florida roadways, and who, in real life, was convicted of and executed for the murders of men who paid to have sex with her.

Well, Wuornos has her movie: In “Monster,” director Patty Jenkins’ “based on a true story” drama, the hooker-turned-killer is portrayed with jumpy and fierce conviction by a virtually unidentifiable Charlize Theron. It’s a guaranteed Oscar-nomination performance: The graceful, glamorous actress from South Africa gained poundage, pasted on a jaundiced patina of fat and freckles, and bit down on a set of stained yellow fake teeth for the part.

But beyond the makeup and thrift-shop wardrobe, Theron is scarily “there”: A scene early in the picture, when she sits at a bar tossing back shots with a swaggering head-jerk motion, is chilling. At first you think, uh-oh, there’s serious thespian emoting going on here, but then you realize that this is who that character is (or was): self-conscious, loud, melodramatic, pickled in alcohol.

Like “Boys Don’t Cry,” which won its lead, Hilary Swank, a best-actress Oscar (and which, in the end, is a better film), “Monster” reimagines the dark, pivotal moments in a life steeped in abuse, sexual confusion, violence and rage. And a life that generated newspaper headlines, evening news reports, and documentary films. (Wuornos is the subject of two, both by the tabloidy British nonfiction filmmaker Nick Broomfield.)

Beginning with a stark image of Theron’s Wuornos huddled beneath a highway overpass in a slashing rain, “Monster” incorporates death-row letters by its subject into Theron’s voice-over narration. Jenkins’ film (her first feature) focuses on the relationship between Wuornos and a young, trouble-prone Midwesterner called Selby Wall, played with tricky, rabbity despair by Christina Ricci.

“Monster” is a love story, really, as Aileen improbably hooks up with the little lesbian girl whose family has banished her.

Theron and Ricci are impressive together, and “Monster’s” strongest scenes happen as the women share moments of scruffy intimacy or hokey Hollywood-style romance (a Ferris-wheel ride, a carrying-over-the-threshold into a new house). But it’s always with the unspoken (and unconscious) certainty that their time together is fleeting, and doomed.

Other aspects of “Monster” are more problematic.

The appearance of Bruce Dern, as a grizzled self-storage-facility manager who befriends Wuornos, does nothing more than remind the audience that there are actors at work here. (The veteran Dern may as well prop up a sign alongside himself to that effect.)

Likewise, the succession of men in cars who have the misfortune to pick up Wuornos, and then get shot for their trouble, becomes a twisted kind of cameo watch: Yup, there’s Pruitt Taylor Vince. Yup, there’s that guy from that TV show. And so on.

To its credit, “Monster” doesn’t romanticize or, in reckless ways, hypothesize. The facts, for the most part, are there, and while references are made to Wuornos’ childhood history of abuse, none of them are intended to serve as an apology for the cold-blooded and devastating murders she committed.

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