Mama Bear revolves around Debra, who is in prison for killing her teenage son, Keith. Keith was a kid-gone-wild, one of the many casualties of interurban poverty who grew up paying more attention to the lessons of the street than to his own mother. Keith was a gangbanger, and his crime resume included robbery, rape, drug dealing, and murder. Debra makes a conscious decision to kill Keith after he and a fellow gang member "accidentally" kill a toddler in a drive-by shooting. (Debra argues the "accidentally" part--in her mind, anytime you fire random shots out of the window of a car as it speeds down the street, you can't cry "accident" if someone is killed.)

However, Debra's decision to kill her own child does not come about as you might think; she's not doing it out of frustration, out of anger, out of retaliation, or out of fear. She's doing it because, as she says, "mamas are responsible for their children," and she thinks it would be irresponsible of her to let Keith continue his destructive life.

Also, Debra is convinced that Keith won't live to see his 18th birthday regardless of what she does, and she refuses to let his last sight be some rival gang member and his last emotions be fear and fury. No, she would rather he die at home, in his own bed, where he feels (at least occasionally) secure and where he regains some of the innocence of his early childhood -- which is why she shoots him in the head while he is sleeping.