"This court has found that Carter's actions and failure to act where it was her self-created duty to Roy since she put him in that toxic environment constituted reckless conduct".
Judge Moniz may have set a risky precedent with his decision, said longtime Quincy, Mass. attorney Bob Harnais.
The victim's aunt Rebecca Maki added that the Roy family is "pleased with the outcome".
"She did not issue a simple additional instruction, 'Get out of the truck, '" he said.
WATCH: Carter sobbed as the verdict was handed down.
The prosecution asked for Carter's bail to be revoked, but Moniz allowed her to remain out on bail until her sentencing on August 3.
She could face up to 20 years in prison. When asked how she would describe what Carter did, Lynn was unable to find the words, saying, "I can not".
Her lawyer argued Mr Roy had a history of depression and suicide attempts and was determined to end his life. When police found Roy's lifeless body in the cab of a pickup filled with carbon dioxide in a Kmart parking lot, it was initially treated as a suicide. In her closing statement, Assistant District Attorney Katie Rayburn said Carter wanted to be seen as the grieving girlfriend.
Roy was apparently having second thoughts about his suicide attempt, but Carter told him to get back into the auto, berating him. "Like I don't get why you aren't", Carter wrote in a text to Roy the day he died. When Conrad told her that he didn't believe in himself any longer, she brought up the fact: 'Take your life?' she questioned.
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"Is she a criminal because she didn't talk him out of it?"
Roy has also attempted suicide previously, the judge noted.
A 20-year-old woman has been convicted of manslaughter for encouraging her boyfriend to commit suicide three years ago.
"She is mindful that the process in the truck will take approximately 15 minutes", Mr Moniz said during Friday's hearing. At the time of Roy's death, Carter was taking Celexa, an antidepressant Breggin said targets the brain's frontal lobe, which controls empathy and decision-making.
What do you think of Michelle Carter's conviction? "You need to just do it".
"If you point a gun at somebody, you're pretty much intending to shoot them", Gutterman said.
The night of his death, Carter encouraged him to get back inside the truck after he revealed he was scared and had gotten out.
But Carter's defenders claimed she was innocent and that Roy's death was a suicide rather than a homicide.