Katherine Waxman

Katherine Waxman (nee Donovan) is the main villainess from "Betrayal," episode 18.11 of Law & Order.

She was played by Moira Kelly.

Backstory
At age 14, Katherine Donovan engaged in promiscuous behavior and began abusing drugs, prompting her mother, Maureen, to send her to a therapist, Dr. Isaac Waxman. During her time with Isaac, they began a sexual relationship (as Isaac was revealed to have had an affinity for teenage girls), and they later married.

Events
In the episode's events, Emma (the Waxmans' young daughter) played a recording of Isaac talking about an affair with another of his patients, as she heard the name "Meredith" mentioned. Katherine heard the recording and went into such a rage that she confronted her husband in his office. During the confrontation, Katherine grabbed Isaac's gun (which Isaac took from a patient who was planning a mass murder, but was talked out of it) and shot her husband to death. She later called her mother while on a boat heading back home; tossing the gun overboard while doing so.

During Katherine's first interview with police, she stated that she had walked her dog at the park and she was home at the time of the murder. However, Emma contradicted the latter half of Katherine's alibi, leading to Katherine being brought in for questioning. She denied the accusations, and following this, Lupo played Isaac's tape--which Katherine quickly stopped. After being confronted about Isaac's affair, Katherine stated that Meredith meant nothing to him, and ominously snarled that Isaac had a sickness and she had to put him down.

Katherine returned with her lawyer, who later revealed that Emma recanted her original claim; suggesting that Katherine coerced her daughter to lie and claim that her mother was home at the time of the murder. After a recording of Katherine's conversation with Maureen contradicted her claim that she was at the park, the villainess is officially arrested and charged with Isaac's murder. Realizing that she's caught, Katherine decided to fire her lawyer and represent herself, and later began portraying herself as a victim of abuse from Isaac, despite the fact that she had told Emma that she loved her husband.

Katherine continued posing as an abuse victim while representing herself and while she was testifying in court, stating that Isaac threatened to send her to a mental institution if she stopped trusting him. During Katherine's testimony, Rubirosa provided more of Isaac's tapes and played one of them--which revealed that "Meredith" wasn't another lover but a town where the institution was. It was also revealed that the recordings Isaac made were about Katherine, his greatest love, and he had expressed that his heart would be broken if Katherine was committed. After realizing that Isaac wasn't cheating, Katherine broke down while repeating the phrase, "I am a rock in a sea of chaos"--the statement Isaac instructed her to say. As revealed at episode's end, Katherine accepted a deal to plead guilty to manslaughter, giving her 15 years to life.