Mary was an "Angel of Death" serial killer working at a hospital where she targeted patients she viewed as "slaves of their own compulsions," such as alcoholics or heavy smokers.

The turning point comes when Dexter witnesses Mary with a new patient: a man suffering from late-stage cancer, begging for just one more day to see his daughter. Mary preps the lethal injection anyway, ignoring his pleas.

The kill was far from the polished, clinical execution seen in Dexter’s later years. It was messy, impulsive, and deeply personal.