What is the difference between Mrs. Maloney and the Landlady?

The main difference between the two ladies lies in the nature of their respective acts of murder. Mrs. Maloney kills her husband in a sudden, unprecedented fit of rage after he tells her he's leaving. There's nothing premeditated about Mary's fatal assault upon her husband. The landlady, on the other hand, commits several acts of premeditated murder upon various young men who come to stay at her bed and breakfast. We're never sure exactly why she does this; her lack of obvious motive makes her crimes all the more chilling and disturbing. Perhaps she's gaining revenge for something bad that happened to her many years ago, perhaps she has mental health issues, or perhaps her unhealthy obsession with taxidermy has just got a little out of hand. We will never know.
But in any case, the landlady's murders are her way of imposing herself upon the world, a highly unconventional means of expressing her eccentric personality. Mary, on the other hand—a meek, unassertive woman—is reacting to the sudden intrusion into her life of a brutal, unforgiving world, one where people such as herself are often hurt and destroyed. Yet by cleverly covering up her murderous act, Mary quickly learns to assert herself, albeit in a way that's much more conventional than that of the landlady.