Karen Campbell (Cynthia Gibb) is the main villainess of "Poison", episode 5.24 of Law & Order: SVU (airdate May 11, 2004). She was the wife of Pete Campbell and the mother of four children, including adopted daughter Alexis. Karen quickly became overwhelmed with the stress of raising four children, and placed blame for that on Alexis.
After unsuccessfully trying to return Alexis to the adoption agency, Karen began abusing the young girl, with her abuse being witnessed by her daughter Katie and sister-in-law Wendy. One day, after Alexis drew on the wall with crayon, Karen forced Alexis to begin washing the wall. When Alexis began crying, Karen became frustrated and shoved the sponge into Alexis's mouth, shoving it back in whenever she spit it out. Alexis became sick and started to vomit, but Karen forced her to stand in the corner and only called the police when Pete returned home from work.
At the hospital, the doctors confirmed that Alexis had been poisoned and the SVU detectives were called in. When questioned by Olivia Benson, Wendy revealed how she witnessed Karen abusing Alexis and how she believed her to have poisoned the girl. Karen was immediately arrested and taken into interrogation, where she proclaimed that Wendy hated her and that she would never hurt Alexis. Despite the evidence against her, including Katie's testimony, Karen was found innocent after the corrupt trial judge Oliver Taft scared Katie into recanting and used his class bias to convince himself Karen was innocent.
During the trial, Taft had a conversation with Karen in the courthouse elevator, during which Karen believed that he said it would be better if Alexis died. Despite Karen's reservations, Alexis was taken out of the hospital and brought back home on Pete's insistence. After becoming angered by Alexis's cries of pain, the evil Karen suffocated her to death with a pillow. Karen was soon afterwards arrested and, after her lawyer Oliver Gates unsuccessfully tried to say Alexis's death was caused by her "accidental" poisoning, confessed to her heinous crime; viciously proclaiming she had just wanted Alexis to be quiet. In the end of the episode, Karen was convicted of Alexis' murder.