Certain patterns trigger Viola's murders.
All of Viola’s on-screen victims were people who represented what she hated, feared, or loved. The Plague Doctor invaded her home by turning it into a hospital and due to his mask looked like the same doctor who delivered the news which ruined her life. The Vicar tried to force Viola to leave her home and she confused the Doll-Face Child with Isabel just as she would later do with Flora. By contrast when Charlotte and Dominic were alive Viola only came to the house once or twice a year and wasn't shown killing anyone, despite the fact she must have seen the couple in bed at least once. Viola also ignored the children and Hannah when she killed Peter, who was stealing from the Wingraves the same way Perdita “stole” from her. Dani is raising the Wingrave children in place of their parents and Henry “seduced” Charlotte away from her husband. Peter briefly considered Rebecca possessing Dani
Peter-through-Miles is clearly attracted to Dani and gives Dani a piece of Rebecca's jewelry the first day they meet. If Rebecca possessed Dani, it would avoid the creepy implications of Peter and Rebecca, a romantic couple, taking on the identities and lives of Miles and Flora, who are brother and sister—even if it would leave a rather disturbing age gap. Peter nixed the idea of Rebecca possessing Dani when he realized Dani would never trust him enough to consent to the possession like the children would. Viola didn't have consumption
While the initial symptoms (coughing, shortness of breath, hemoptysis) are very similar, the fact that Viola survived for nearly five years after her diagnosis suggests that she actually had a form of lung cancer, not tuberculosis. This also explains why Perdita never caught Viola's illness from her, despite tuberculosis being extremely contagious and easily spread by contact with an infected person's bodily fluids. If she did have cancer, it might have eventually metastasized to her brain, which would explain her rapidly increasing paranoia and unusual violent behavior in the final months of her life.