Victim name: Mary, AKA Angel of Death
Age/Occupation: 45, Nurse
Where/how Dexter stalks victim: Dexter broke into Mary's house and ambushed her, sticking her with a syringe, like the one she used on his father. (This is the origin of Dexter's syringe technique.) But Mary was strong. She took the needle out of her neck and wrestled Dexter to the ground. Finally Dexter brought her unconscious with a punch to the face.
Why victim deserves it: Harry's testimony. Mary was Harry's nurse while he was hospitalized for a heart attack. She administered his morphine and attempted to kill him by overdose. Harry realized what was happening and asked Dexter to 'stop her...before she [hurt] anyone else.' Harry gave Dexter permission to use the lessons he was taught.
Victim's victims: Phoebe Burns (60), Jason Kheel (82), David Nammers (73)
Dexter's kill methods and rituals: Because it was his first kill, Dexter went slightly overboard, covering not just the kill room, but the entire apartment, in Painter's plastic. Mary woke up shrink wrapped to a kill table, naked, with gauze in her mouth. Dexter perused her scrap book of obituaries, removed the gauze, and ask her about her victims.
Victim's last words: "I helped them... I took away their pain."
Dexter's last words to victim: "I understand. Now it's time to take away your pain."
Location of killing: Mary's living room.
Close calls/slip-ups: While striking Mary with the syringe, he doesn't fully eject the contents into her; thus her pulling it out and them getting into a brawl.