Mary Maloney is the protagonist and her husband, Patrick Maloney, is the antagonist. Mary is the main character, around whom the story revolves, and this makes her the protagonist. The "pro" of protagonist suggests a positive quality and while many protagonists are the "good guys," "good girls," or heroes, this is not always the case; obviously, Mary's heroism is questionable to say the least. The protagonist is simply the main character: Mary. We can say that Patrick is the antagonist because he is the one who antagonizes Mary.
However, by that definition, a reader could contend that Mary is an antagonist as well because (although Patrick is not the protagonist) she opposes Patrick and/or antagonizes him (to death) when he reveals that he is leaving her. But in typical readings, Mary would be the protagonist and Patrick would be the antagonist. The policemen are also potential antagonists in the sense that they pose potential opposition to Mary when they come to investigate the murder. But in Mary's brilliant strategy, she turns them from potential antagonists to allies by making them unknowingly eat the evidence:
"That's a big bar the murderer must have used to hit poor Patrick. The doctor says the back of his head was broken to pieces.
"That's why the weapon should be easy to find."
"Exactly what I say."
"Whoever did it, he can't carry a weapon that big around with him."
"Personally, I think the weapon is somewhere near the house."
"It's probably right under our noses. What do you think, Jack?"