While this question is open to interpretation, 5 crucial moments that develop and resolve the conflict can be identified. This is how I see them:
1) Patrick drains his drink, and refuses dinner. Mary becomes aware that there is tension because of this.
2) Patrick announces his desire to leave his pregnant wife.
3) Mary kills her husband with the frozen leg of lamb.
4) Mary calls to police to announce that she has "found" her dead husband.
5) The police eat Mary's dinner, including the leg of lamb, and theorize about the murder weapon.
Dahl really ends on an ironic note here, with the investigators not aware that they are eating the murder weapon. Readers are left with the understanding that Mary will be spared.
"Lamb to the Slaughter" is a fairly short story so to choose five important scenes would pretty much encompass the entire work. The first important scene is when the husband comes home. The details of the scene are significant because they build the tension as Mrs. Mahoney realizes there is something wrong, and the husband finally confesses what it is. Another significant scene is obviously the murder itself. Then there is the scene at the grocers where Mary acts in complete denial to what she has just done. Next, there is the 'discovery' of the body and the investigation. And finally, the ironic scene at the end where the police are eating the evidence while discussing the evidence without even knowing it.