Then, if she lied about the tap, it would be to cast suspicion away from her. She has no alibi for 2:30 to 3:00, and her room is right next to Lydia's. She starts the tub at high power, drowns Lydia, and by the time she 'discovers' the crime at 3:20, the tub is already nearly overflowing.

...I suppose that's something to keep in mind. We must examine all the testimony in turn as if each subject were lying. What information do we have that is given only by Miss Marlow or by Bernkastel or by myself? All of it must be considered in the light that we have only a suspect's words to go off.