Aileen Wuornos (referred to as Lee throughout the movie) did not have a happy childhood.
In the film version, she was repeatedly raped by a friend of her father's from the age of eight. Her father then not only disbelieved her, but beat her for it too. Pregnant at thirteen, her baby was put up for adoption. It's implied that she had no choice in that.
A youngest child, she nevertheless was asked to support her brothers and sisters, once they were all abandoned by their father. She did this, from thirteen years old, by working as a prostitute. Her siblings disowned her when such information became common knowledge.
Her hard life on the streets, supported by prostitution, was close to ending in suicide, when a chance meeting brought Selby Wall into her life.
A lesbian under severe pressure from her devoutly Christian family to conform, Selby was the first person who really loved Aileen. So much so that Aileen was prepared to go to extreme lengths to provide for her lady. Like get a job.
Unfortunately, the job market is neither nice nor welcoming to street girls trying to go straight. Desperate straits brought desperate measures; and choices from which she could not retreat.
Monster is a hard-hitting and powerful story, which doesn't stint on the harsh realities of cussing, sex and violence. The latter is not gratuitous, but it does tell it as it was in Aileen Wuornos's life. At least in the close version of it that is depicted here.