Answer:
The guilty party at the end of the story is Lizzie's father John.
John's neglecting of her daughter Lizzie in her childhood contributes to the story's theme of guilt, responsibility, betrayal, and redemption.
Step-by-step explanation:
"The Guilty Party" is O. Henry's short story from 1909.
In the start of the story it is narrated through third person point of view, and some times by Lizzie's first person narration that Lizzie as child loves her father a lot. But her father keeps neglecting her, and does not give her time. It is apparently because he is busy with his business but on deeper level, he representing a typical father in a male dominant society of late nineteenth and early twentieth century, does not want to spend time with his child.
In the next part Lizzie as an adult is engaged to Kid, but he too behaves just like her father. He betrays Lizzie by dancing and spending more time with Annie. This frustrates Lizzie who as an act of redemption kills Kid.
The court officer wants to give Lizzie a benefit. He wants Lizzie to be judged based on the real cause behind her action. He considers Lizzie's father a real guilty party, for neglecting his child when she needed his time and attention needed to guide her in proper and appropriate manner.