I think the part that I find so fascinating about Mary Maloney's transition from the doting-wife protagonist to the murder-covering antagonist is that the transition is so understandable. Her entire world revolves around Patrick, and his announcement hits her hard. It's a completely unexpected blow. And Mary sits there and takes it. She even decides to cook dinner for the guy. No yelling. No slap to the face. No walking out of the house to vent to a girlfriend. Nothing.
But anybody who has ever had some pent up aggression and/or frustration knows it has to come out somehow, sometime, and somewhere. Patrick's announcement that he isn't hungry is the last straw for Mary. This time she delivers the unexpected blow . . . literally. I still don't think of her as a bad guy at this point, though. Her action doesn't seem out of place. The following actions, though, get more and more devious and antagonistic. She's immediately plotting how to get away with the murder. She cooks the lamb, goes to the store to establish an alibi, comes home, acts shocked to find a dead body, and calls the police. Still, at this point I'm not totally disgusted with Mary. I'm horrified, though, at the end. She feeds the murder weapon to the police and laughs at the fact that the murder weapon will never be found. Her laugh is either a giggle of joy or a sinister chuckle. Either way, she's got no remorse in my opinion, which makes her transition to antagonist complete.
"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?"
And in the other room, Mary Maloney began to laugh.