Rhonda Carnegie was the wife of Judge Alan Carnegie.
Unknown to her husband, Rhonda was carrying on an extra-marital affair with a man twenty years younger than her. Knowing that if her husband found out, she would face a messy and bitter divorce, she decided to kill him first.
She saw her opportunity when mafia boss Salvatore Lucarelli was finally indicted. Because Alan was the designated alternate to hear Lucarelli's case, Rhonda killed both her husband and the first judge, Clarence Stanton, to make both murders seem like Lucarelli's work.
Both men were killed in similar fashion: she followed them to a public park on her bicycle and shot them with a handgun. Her husband happened to be walking their dog at the time, and the fact that the dog didn't bark - meaning it knew the killer - was a telling detail that Adrian Monk noticed.
When the police visited her home, she pretended to be broken with grief, but a few quick observations by Monk convinced Captain Stottlemeyer to hold her for questioning and get a search warrant. He predicted that she might have had time to get rid of the bicycle and the clothes she wore, but not the gun. Incredibly, Stottlemeyer was wrong: such was Rhonda's hubris that she had not yet bothered to get rid of any of them, and they were turned up by the search warrant, along with her motive for killing both men.