In "Lamb to the Slaughter," a male stereotype of women works to Mary's advantage in two ways. First of all, Patrick stereotypes women as being concerned only with financial and material comfort. He refers to this when he announces his intention to divorce Mary:
Of course, I'll give you money and see that you're taken care of. But there really shouldn't be any problem.
As such, he does not think that Mary will react violently to his announcement. It is ironic, then, that Mary reacts in the most violent way possibly by murdering him.
After Mary kills Patrick, a male stereotype of women as gentle and non-threatening prevents the police detectives from viewing her as a potential suspect. We see direct evidence of this stereotype in the way the detectives treat Mary. They speak to her "gently," and are "exceptionally nice" to her. Their tone is patronizing when they explain Patrick has been killed by a blow to the head. Again, this is another example of irony because Mary knows too well how Patrick died since she was the person who killed him.