Detailed Explanation
Guinevere Beck was murdered by Joe Goldberg in the Season 1 finale. After discovering Joe's hidden lair beneath Mooney's bookstore—complete with his stalker collection of her belongings and evidence of his murders—Beck tried to escape. She briefly freed herself and made it to the street, but Joe caught her and brought her back to the cage. In a devastating scene, Joe strangled Beck to death, all while convincing himself he had no choice. He then framed her therapist Dr. Nicky for her murder using Beck's phone and planted evidence, resulting in Dr. Nicky's wrongful imprisonment. The murder set the template for Joe's pattern: obsess, "protect," destroy.