In part of Judge Moniz's reasoning, the judge said that in addition to her texts, Carter had urged the boy during a phone call to get back in the truck that was being filled with deadly poison and that she did not alert authorities that Roy was taking his life.
Text messages sent by Carter after Roy's death implied she listened to Roy die over the phone and convinced him to go through with the suicide when he got cold feet.
"She admits in subsequent texts that she did nothing, she did not call the police or Mr. Roy's family".
Carter, now 20 but 17 years old at the time of Roy's death, waived her right to a jury trial, meaning that members of the community did not render the decision; Moniz acted exclusively as jury. Prosecutors argued that Carter wanted Roy to die so she could play the part of the grieving girlfriend.
Carter waived her right to a jury trial.
Prosecutors had alleged Carter, then 17, encouraged Roy in text messages.
Carter will be sentenced on August 3, and could receive 20 years to spend in prison.
The judge ruled that Carter, now 20, can remain free on bail but ordered her not to make any contact with Roys family or leave the state.
But during the trial Carter maintained her innocence. At no time did she tell him to seek help for his feelings about killing himself, but instead talked about the different ways to do it.
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The judge noted that the 18-year-old Roy climbed out of the truck as it was filling with toxic gas and told Carter he was scared.
Carter's attorney, Joseph Cataldo argued Roy was simply forcing Carter to be part of his second attempt.
The court heard more than 20,000 text messages exchanges were made - with more than 1,000 of those messages sent in the days leading up to Roy's death.
According to The New York Times, many legal experts expected Carter to be acquitted, but the eventual verdict suggests that on a legal basis, someone's words alone can now be interpreted as the direct cause of another person's suicide.
Judge Lawrence Moniz found that Carter forced Roy to get back in his truck.
"This court, having reviewed the evidence, now finds you guilty on the indictment charging you with involuntary manslaughter", Mr Moniz told the court. You need to do it like you did last time and not think about it and just do it babe.
On June 16, Moniz delivered a guilty verdict for involuntary manslaughter.
The ACLU's MA chapter says in a statement that the conviction "exceeds the limits of our criminal laws and violates free speech protections guaranteed by the MA and U.S. Constitutions".