To compare and contrast the two murderers in Roald Dahl's two short stories "Lamb to the Slaughter" and "The Landlady," you'll want to compare and contrast their characterizations by thinking about what they are like as characters, what kinds of things they said and did, and what your overall impression of the two characters is. One thing of importance to note is that the murder in "Lamb to the Slaughter" was not premeditated; it was an act of passion she barely realized she was doing. In contrast, the murder in "The Landlady" was very much premeditated, right down to specifically choosing exactly what the young man she was waiting for would look like. Since Dahl presents one murder as premeditated and the other as not, Dahl also creates an underlying theme of the presentation of innocence that can also help with your analysis.
In "Lamb to the Slaughter," the murderer Mary Maloney is characterized as being very innocent. Many elements in Dahl's narrative describe the scene as innocent. The room is described as being very innocently warm; two table lamps are very innocently lit; and two glasses are even very innocently waiting on the cupboard. What's more, she is described as an expectant mother, waiting for her husband to come home, innocently sitting and sewing and feeling "curiously peaceful." She even greets her husband with, "Hello, darling," and continues to offer her assistance, just as usual. She is also described as barely knowing what she's doing when she fetches a frozen leg of lamb to fix for supper and whacks her husband over the head with it. All of these details serve to characterize her as a very innocent woman who is emotionally abused by her husband and incited to act out violently when told he was leaving her while pregnant.
In contrast, Dahl's descriptions in "The Landlady" lead the reader to be suspicious from the start. The landlady of the bed and breakfast also has her fair share of innocent descriptions, such as her "warm welcoming smile," her "round pink face," and "gentle blue eyes." Yet she also does her share of suspicious actions, such as saying Billy's room is already prepared for him as if she was expecting him, watching and waiting for him. She does many other suspicious things too, such as say she's very "choosy and particular" about who she accepts as guests. All of her suspicious descriptions serve to paint her as an insane and premeditating murderer as opposed to Mary, who innocently fell victim to mistreatment and lashed out.