How is Mary the protagonist in Roald Dahl's story "Lamb to the Slaughter"?

Mary is the protagonist of "Lamb to the Slaughter" because she is the central character that the story revolves around. While I might be able to argue that Patrick drives the early events of the story, Mary is the character who moves the story toward its climax. Patrick simply isn't alive long enough to be considered a protagonist in the story. The other characters, in addition to getting very little page time, react to Mary and her plan. They do not drive the action of the story. From the point where she clubs Patrick to death with the leg of lamb, Mary is in control of the events that follow. She comes up with, executes, and she gets away with her plan.  
It's also common for a story's protagonist to be someone a reader would root for. Deep down, I think most readers know Mary is getting away with murder/ manslaughter. She's getting away with it through deception, too, although that probably doesn't bother readers as much, if at all. Mary is a protagonist because we are secretly cheering for her and her plan. Every time I read this story, I still hope Mary gets away with it. I sympathize with her and her situation, and I don't want to see her punished for what she did. She's the "good guy," and good guys are usually protagonists. Bad guys are antagonists.
https://literarydevices.net/protagonist/