Karla

The film starts with a framing device of psychologist Dr. Arnold (Patrick Bauchau) in a session with Karla Homolka (Laura Prepon) on May 11, 2000, at Canada's Regional Psychiatric Centre in Saskatoon, Saskatchewan. Dr. Arnold's evaluation will determine Karla's eligibility for parole. During the session, Dr. Arnold shows Karla a photo album of herself and her husband Paul Bernardo (Misha Collins) in happier times, and Karla has a memory of how they first met. However, when Dr. Arnold introduces the subject of Karla's sister Tammy, Karla becomes uncooperative. Over the following weeks, Dr. Arnold probes Karla about her knowledge of Paul's secret life as a serial rapist, and it is revealed that shortly before their marriage Paul began to rape women. Paul convinced Karla to help him rape her sister on camera. At first Karla disagrees, but eventually steals Halcion from the veterinary clinic where she works. Tammy gets drunk at a Christmas party and becomes unresponsive after the two drug her. Tammy is raped, and ends up getting sick while drugged, choking on her own vomit. Karla calls 911 and the two hide the assault evidence so her death would appear accidental. Afterwards, Paul beats Karla for "ruining his movie". Karla learns never to ruin Paul's "movies".
Karla tells Dr. Arnold that Paul became obsessed with Tammy and kept watching the video after her death, even showing it to friends. Paul also threatened to reveal Karla's role in her sister's murder if he should get into any more trouble, but Karla finally leaves him anyway.
Paul is open with Karla about his crimes, yet Karla does not object to them, even when Paul begins to bring his victims home. Although disturbed at that part of Paul's life, Karla learns to accept it, knowing that she too will be killed if she does not. At Paul's request, Karla hesitantly participates in these assaults. Paul kidnaps a young girl, Tina McCarthy, and forces her to undress on camera. Karla witnesses this, but is too afraid to tell anyone. Tina is held for 3 days, and is raped repeatedly while Karla is forced to videotape it all. During one such assault, Paul strangles Tina after she sees his face. Karla tries to convince him not to kill her, but he insists, telling her that Tina can identify them both. He cuts the body into pieces and seals them in cement blocks, which he then dumps into a lake. On the day of Paul and Karla's wedding, the girl's body is discovered, and identified by dental records. Paul stops raping and abducting for a time, but his anxiety and pent-up frustration cause him to become violent toward Karla. Paul's friends see the change in his personality and break off from him. Meanwhile Karla, suffering from the abuse and desperate to reclaim his affections, helps Paul abduct and rape another young girl, "Kaitlyn Ross" (based on Kristen French). Kaitlyn is brutally raped by Paul. Karla watches the assault and begins to cry. Paul beats Karla and tells her that she is ruining his movie again. After Paul leaves, Karla is put in charge of Kaitlyn. Kaitlyn tells Karla that she is a victim too, and that this is not her fault. When asked why she stays, Karla simply says, "You don't understand". Karla explains to Dr. Arnold that she wanted to free Kaitlyn, but was afraid Paul would kill her if she did.
Kaitlyn's disappearance immediately attracts a storm of media and police attention, so Paul kills Kaitlyn to be rid of her. As part of the investigation, the police arrive at Paul's house. Paul is very cooperative so the police leave satisfied, but, afterwards, he beats Karla mercilessly. Karla takes refuge with friends before reporting the assault. Paul is then booked under domestic violence but is released after only one night in jail.
Back at the Regional Psychiatric Centre, Dr. Arnold asks Karla about her relationship with her younger sister (Tammy Homolka) and her jealousy towards Tammy because of Paul's attraction to her. Karla confesses that Paul wanted to take Tammy's virginity and that he wanted Karla to "give" Tammy to him. Karla agrees to help him rape her sister.
Shortly thereafter the DNA samples Paul provided as part of the Scarborough rape investigation are matched to the evidence found on one of the murder victims. Both he and Karla are subsequently arrested. During the trial, Paul testifies that Karla killed Kaitlyn Ross with a mallet. Karla denies all the killings. Paul is convicted on two counts of murder without any possibility of parole. Karla is given a sentence of twelve years in exchange for a guilty plea for manslaughter.
The film ends with a note explaining that Karla's parole was denied, as the psychologist found her artificial, manipulative and without remorse.

Question: Who killed Kaitlyn?
Answer: Paul