Because Mary was Dexter’s first kill, his method was notably less refined than his later work:
This creates a unique narrative paradox where Dexter must use the language of a serial killer to debunk the language of a savior. dexter kills nurse mary
When we see the flashback, Dexter is young, unskilled, and messy (he forgets the plastic wrap, the room isn't fully prepared). This kill is the Because Mary was Dexter’s first kill, his method