In "Lamb to the Slaughter," what are three character traits that describe Mary Maloney (be it the beginning or the end of the story)?

Mary Malone's character is well-described through direct characterization all throughout the story "Lamb to the Slaughter." This direct characterization occurs each time the author describes Mary physically and in detail. An example of direct characterization is 

Her skin -for this was her sixth month with child-had acquired a wonderful translucent quality, the mouth was soft, and the eyes, with their new placid look, seemed larger darker than before.

To extrapolate character traits,...

Mary Malone's character is well-described through direct characterization all throughout the story "Lamb to the Slaughter." This direct characterization occurs each time the author describes Mary physically and in detail. An example of direct characterization is 

Her skin -for this was her sixth month with child-had acquired a wonderful translucent quality, the mouth was soft, and the eyes, with their new placid look, seemed larger darker than before.

To extrapolate character traits, you can use direct characterization as well as indirect characterization. Indirect characterization occurs when the author describes behaviors and reactions in the character. From such behaviors and reactions, the reader can deduct more facts about the character than the author lets on. 

An example of indirect characterization is: 

She knew he didn't want to speak much until the first drink was finished, and she, on her side, was content to sit quietly, enjoying his company after the long hours alone

From this excerpt, you can deduct that Mary is submissive to her husband. This is obvious from the way that Mary waits on her husband, lets him take the lead on things, and in the way that she even feels happy to be taken care of. 

A second character trait you can get from Mary is that she is clever. The way that she was able to figure out how to get away with murder shows someone who thinks quickly and understands how to avoid consequences. 

A third character trait is that she is evidently, at one point, terrified. While her demeanor after the crime shows a woman who has dissociated from what she has just done, it is obvious that part of the reason why she snapped and killed her husband is the fear of being left alone, with a child, and semi-destitute. Obviously, she would have not reacted in such a drastic way if she didn't have a good reason for it. Fear is the biggest reason to react instinctively.