There’s no excuse for murder. But there are always circumstances, and these are shrewdly explored in this biographical drama of American serial killer Aileen Wuornos, a hard-luck prostitute in Daytona Beach, Florida, who killed six men—all of them her clients—in 1989.

Written and directed by rookie filmmaker Patty Jenkins, the movie tells a tough story with measured and persuasive empathy. It also features a fierce, startling performance from Charlize Theron as Wuornos. Packing a few dozen extra pounds, her skin weathered from hard living, her face doughy and her hair lank, Theron’s physical transformation alone is enough to earn her an Oscar, but it’s the ferocious credibility of her performance that propels the movie. Swaggering and volcanic, yet tender, cynical yet hopelessly optimistic, she’s a brutalized Everywoman driven to believe she has no choice but to fight back.

Jenkins shapes the material as a love story between two outcasts and uses a series of remarkable letters Wuornos wrote to a friend during her 12 years on Death Row (she was executed in 2002) to provide a wistfully detached narration. Recalling girlish dreams of being “beautiful and rich” at odds with a childhood of abuse and neglect, the adult Aileen is contemplating suicide under the freeway overpass where men pick her up for sex in their cars. Instead, she decides to spend her last five bucks on a beer, wanders into a nearby gay bar, and meets timorous Selby (played by Christina Ricci as a passive-manipulative innocent), a middle-class mid-Westerner banished to relatives in Florida to “cure” her fledgling homosexuality.

Drawn together by mutual need, they run off together on an odyssey through a series of bleak motels. One night a homicidal john rapes and tortures Aileen, who wrestles free and kills him in self-defense. Shaken, she decides to quit hooking and get an office job (with predictable results, in a sequence of comic asperity). Reluctantly going back to hooking to support Selby, Aileen finds that killing the men who use her is a hard habit to break.

At no time does Jenkins ask us to forgive Wuornos’ brutal crimes (the murder scenes are stark and unflinching), or urge us to identify with her. But Jenkins does make comprehensible the hard-bitten milieu of bars, motels, freeways, and fast sex that Wuornos inhabits, and the percolating rage that once unleashed becomes impossible for her to resist.

Monster [3 stars]

Directed by Patty Jenkins

Starring Charlize Theron, Christina Ricci, Bruce Dern

(Rated R 111 mins.)