HOLIDAY — Warren Birkbeck went to prison in 2000 for slashing his estranged wife's throat in a New Hampshire condo. Decades earlier, authorities say, he was convicted of manslaughter in Massachusetts. Now, two years after his release from prison, the 71-year-old man is accused of killing a third person, this time in Pasco County. Birkbeck was booked on a charge of premeditated murder Wednesday after telling deputies that he stabbed a female roommate three to four times in the chest. He explained that he was in love with the victim, 42-year-old Denise Cook, but that she didn't feel the same way. "I just have a hard time being rejected," he said, according to the Pasco County Sheriff's Office. The attack happened just after 3 a.m. Wednesday in a home on Trask Drive in Holiday that was shared by three unrelated adults. Cook died at the scene. Birkbeck was held without bail at the Pasco County Jail. He was still on probation from the New Hampshire killing, which, after a plea agreement, produced his second manslaughter conviction. "There's no other way to say this: He's a killer," Pasco County Sheriff Chris Nocco said at a news conference, where he listed Birkbeck's past convictions. "He has a history of killing people." An Associated Press report from 2000 details the New Hampshire stabbing. The circumstances of the Massachusetts conviction were not immediately available. Birkbeck moved to Florida two years ago after his release from prison to live with his sister, also on Trask Drive. After his sister died, neighbor Lillian Meyer, 85, took him in, the sheriff said. The church organist, known for her willingness to help others, also took in Cook. A friend said the two women attended church together. Birkbeck has been under the supervision of the Florida Department of Corrections since Oct. 14, 2015, state records show. He has checked into his local probation office monthly since then, according to agency press secretary Ashley Cook, who called the event "tragic." She said he last met with a DOC officer on July 12, and that an officer had previously visited the residence unannounced. Holiday neighbor April Sullivan, 42, said this is the first she's learning about Birkbeck's past. "I've never seen a parole officer pull up in front of that house," she said. "Who's watching him?" The neighbor said she felt uncomfortable around Birkbeck. Something felt off the few times he would stop to chat or try to give her bags of old clothes. Now she wants to know why others who knew Birkbeck's past didn't seem more concerned. "This is a system failure issue," she said. Neil LaRose, a longtime friend of the victim, said he knew about Birkbeck's New Hampshire manslaughter conviction but not the one in Massachusetts. He visited the home in Holiday often and said in recent weeks Birkbeck showed signs of mental health issues that should have been addressed. "I truly don't believe they kept tabs on him enough," LaRose. Under terms of his release, Birkbeck had been ordered to participate in a substance abuse program, avoid alcohol and obtain mental health counseling, among other restrictions. He was also barred from unsupervised contact with children. Aside from a criminal registration in February of this year, he had no recent criminal history in Florida, state records show. Deputies said Wednesday's incident began after Birkbeck overheard Cook talking to a man on the phone. He entered her bedroom to confront her. She screamed at him to get out. Birkbeck left the room, went to the kitchen, grabbed a knife and returned, the Sheriff's Office reported. Afterward, he entered the older woman's room and told her he had just stabbed Cook and that he believed she was dead. They called 911 together — the 85-year-old organist and the man now accused of murder. Birkbeck got on the phone and described his actions, the Sheriff's Office said. He said in an interview with deputies that he "just flipped out." Cook was a mother of three, though her children did not live with her. Her friend LaRose said meeting Meyer was a turning point in Cook's life, after a rough stretch. "She's been down on her luck," LaRose said. The women had known each other for at least a year. With Meyer's help, Cook rediscovered her faith in God, LaRose said. "Denise and I used to call her [Meyer] our angel," LaRose said. LaRose recalled years long ago when Cooke was "super mom" of her Clearwater neighborhood. Cook would host pool parties and deck out her house in thousands of Christmas lights, reflecting a personality that was always cheerful, smiling and bubbly. Her kinship with the church organist planted sparks that reminded LaRose of that old self. The older woman pushed Cook to get her life back on track, he said. Cook was about to start a job as a medical assistant. None of them considered Birkbeck's interest in Cook any more dangerous than a crush, LaRose said. But now he is mourning the loss of a friend who not long ago was sharing optimistic plans for the future. "It's just a shock and a major tragedy," he said. Senior news researcher John Martin contributed to this report. Contact Chris Bowling at [email protected] or at (813) 435-7308. Follow @chrismbowling.