The nurse was arrested because police believed that

A nurse has been arrested on suspicion of murder after the deaths of three patients at a hospital in Stockport, Greater Manchester. Rebecca Leighton, 27, of Heaviley, Stockport, was arrested at home and is being questioned after two men and a woman died at Stepping Hill Hospital.
    Sixty detectives have been involved in the inquiry and officers have so far questioned more than 50 people.(30)Police have been stationed outside Ms. Leighton’s flat on Buxton Road, where earlier officers removed six bags of items and a computer. The Nursing and Midwifery Council said proceedings were under way to suspend her nursing registration while she is under investigation.