Thomas Marchek is the rapist and mutilator of a little girl, Corrine Wilson, leaving her post-traumatic and homeschooled. Elliot Stabler arrested Marchek, and Marchek eventually started dating Gloria Palmera once released.
Marchek deliberately infected Palmera with HIV, and thus her friend Louise Durning, who was a social worker and dying from AIDS, shot Marchek dead. When Palmera tried to kill herself and confessed to the murder, Durning stepped up and told the truth, about Marchek's and fellow HIV-infected rapist Craig Moss' murders. (SVU: "Victims")