Why is Ellen the thief in mystery at the termond estate?

In "Mystery at the Termond Estate," Ellen is revealed as the thief due to her motive of financial desperation. Throughout the story, she displays knowledge of the estate's layout and the timing of its security measures, which suggests premeditation. Additionally, her interactions with other characters raise suspicion, as she often diverts attention away from her activities. Ultimately, her actions are driven by a need to resolve her own financial difficulties, leading her to commit the theft.