Sandra Birchmore, 23, texted a friend the news in late January 2021. Farwell, then a police officer in Stoughton, Massachusetts, near Boston, had visited Birchmore’s apartment the night before, according to an affidavit accompanying Farwell’s indictment and arrest for killing her. She thought Farwell bringing soda might be a sign that their relationship was improving. “He’s coming around,” her friend texted. “A lot more than I thought he was going to yes,” Birchmore wrote back. His behavior toward her that night had been markedly different, a “complete 180 from where we were at,” Birchmore texted another person, according to...