The evil Ruth Collins
Ruth Collins was a villainess from "No Regrets," episode 3.18 of Blue Bloods (airdate March 15, 2013). She was one of three murder victims in the episode, having been found shot to death in a retirement home.
Detectives Danny Reagan and Maria Baez found Ruth's body at the home, and afterwards, one of the tenants stated that Ruth had a secret: while drinking, Ruth had revealed that she killed someone. Danny's sister, Erin Reagan revealed that Trevor Holt (Erin's mentor) prosecuted the case, and it was their meeting with Holt that revealed Ruth's actions.
As it turned out, Ruth was a slumlord, and she was confronted by one of her tenants, Joan, who was disabled and had threatened to sue. Ruth turned heel in response by running over Joan in her car, and after doing so, the villainess backed up and ran her over again, killing her instantly. She was arrested for the murder, but on during her trial, the evil Ruth portrayed the persona of a sweet elderly lady, claiming that she was foggy due to her medications being mixed up. Ruth's manipulations resulted in an acquittal, and as Holt stated, Joan's son, Kenneth, became angry and vowed to get revenge. However, it was revealed that Holt was the true killer, as all three of his victims were people he prosecuted in his career, and after learning that he was dying of pancreatic cancer, he decided to enact a murderous revenge by shooting Ruth and his other two victims to death.