 -- The aunt of Conrad Roy, a Massachusetts teenager who died of carbon monoxide poisoning after locking himself in his truck, says Roy's then-girlfriend, Michelle Carter, who urged him to commit suicide, "has a damaged moral core."

"I don't think that she helped him kill himself," Roy's aunt, Kim Bozzi, told ABC News' "20/20" ahead of Carter's sentencing today. "I think she forced him to kill himself. I think she was responsible for his death."

Roy was 18 when he died in July 2014 after locking himself in his truck.

When Carter, who was 17 years old and Roy's girlfriend at the time of his death, went on trial this summer, the prosecution argued that she was reckless and caused Roy's death by telling him to get back in the car even though prosecutors said he didn’t want to die.

"I could’ve stopped him," Carter texted a classmate two months after Roy's death, according to testimony. Carter texted that she and Roy were on the phone the day of his suicide when Roy "got out of the car ... he was scared." Carter texted that she "told him to get back in."

Carter's defense claimed that she had previously tried to talk Roy out of harming himself, pointing to one conversation where Roy told Carter he regretted dragging her into his plans to kill himself.

Bozzi told "20/20" when she read the text about Carter telling Roy to get back in the truck, "My heart broke because, unfortunately, he did."

Bozzi said the most "unbelievable" part of Carter's actions was "how she acted after the fact. She was there, sitting on the phone, talking to him while he was taking his last breath."

"Then she texted my niece a couple hours later, 'Hey, do you know where your brother is?' Then she texted his mom the next day, 'Oh, hey, have you heard from Conrad?' Knowing all along," she said.

In June, Massachusetts Judge Lawrence Moniz found Carter guilty of involuntary manslaughter, describing her behavior as "reckless." While announcing the verdict, Moniz said that Carter instructed Roy "to get back into the truck well knowing of all of the feelings he [had] exchanged with her, his ambiguities, his fears, his concerns."

Carter today was sentenced to 2.5 years in the Bristol County House of Correction.

Bozzi wrote a letter to the judge in June ahead of Carter's sentencing, asking for the maximum possible sentence of 20 years.

"Twenty years may seem extreme but it is still 20 more than Conrad will ever have," she said.

"I believe she should be kept far away from society," Bozzi wrote. "Take away the spotlight that she so desperately craves."

Bozzi wrote to the judge, "The defense said it was inevitable, that he would take his life eventually with or without her. I don’t believe so. What I do believe is if it weren’t Conrad then it would have been another boy, from another family, in another town."

Bozzi wrote that Carter "preyed on his vulnerabilities, he trusted her, which in turn, cost him his life."

Bozzi wrote, "The heartbreak over the loss of Conrad goes without saying, our family is broken. ... Although he battled depression he was working toward getting through it. There was light at the end of the tunnel but she snuffed it out."

While Bozzi said "I don’t believe she can be helped," Carter's father, David Carter, pleaded for probation and counseling in a July letter to the judge, according to The Boston Herald.

David Carter wrote in a letter obtained by the Herald, "I pray to God you will take into consideration that Michelle was a troubled, vulnerable teenager in an extremely difficult situation and made a tragic mistake."

“She will forever live with what she has done and I know will be a better person because of it,” he said. “I ask of you to invoke leniency in your decision-making process for my loving child Michelle.”

Bozzi told "20/20" she wants people to know that her nephew "wasn't a troubled young teen, that he wasn't suicidal. ... He did struggle. He did have depression, he did have social anxiety, and a lot of people do. A lot of boys do. A lot of people don't like to admit it when you have that, because you think it's a sign of weakness, so you don't like to share it. But it's okay."

"Mental illness needs to be further researched and treated," she said. "As far as violence against men, I think is something that gets swept under the rug. I think women bully just as much."

Bozzi said she went to court every day of Carter's trial for her nephew.

"There's nothing else I can do for him," she said. "I know that I know how much he loved his mom and his sisters, and he's protective of them. I just try to watch over him like I know he'd want me to."

"I think the world gained an angel. I think that hearing his story and getting to know who he was, I think he's in his absence just making a huge impact on people's lives," she said. "If it can help a couple people then he's happy. I can see him smiling."

ABC News' Joseph Diaz contributed to this report.