Cindy Marsh (Pamela Bowen) is the main villainess who appeared in the bookend episode of Murder, She Wrote "O'Malley's Luck" (S06E18). She is the secretary of businesswoman Gretchen Trent.
It turns out, Cindy was having a love affair with Gretchen's husband Roland Trent, who was another businessman. This led to a dilemma — Roland and Cindy wanted to be together, but if Gretchen divorced Roland or vice versa, she would take her share of the company and reveal the affairs Roland had. So murdering Gretchen was the option.
Roland had prepared to commit murder by taking the private elevator to the office while Cindy walked out of the building through the front door. But Cindy wanted to deal with Gretchen herself, she went upstairs to negotiate with her regarding the affair and the share of the company, as she did not want to jeopardize the company share. This negotiation got out of hand and it led to Cindy grabbing the bookend and bludgeoning Gretchen to death. Cindy, panic-stricken, escaped through another elevator. Roland arrived to see his wife dead, he was too late to kill Gretchen. Since he knew Cindy killed Gretchen, he staged the murder as a suicide by throwing her body out through the balcony.
Lt. Jim O'Malley, a good friend of Jessica Fletcher, revealed Cindy as the murderer through a demonstration — as Cindy was attempting to escape Lt. O'Malley's accusations, she went to the elevator and clicked the button to that elevator. How did Cindy know which button controls which elevator if she claimed that she has not been in the office before? Unless, Cindy had killed Gretchen and used the elevator as an escape route. Cindy confessed to the murder and was arrested off-screen (only the recording of her confession was heard).