Her sentence could range from probation to 20 years in prison.
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.
Lynn also said that she doesn't think Carter has a "conscience".
Defense attorney Joseph Cataldo portrayed Carter as not in control of her actions because of prescription medication that left her with the delusion that she could help Roy by urging his death.
Carter's school friend Samantha Boardman testified that Carter had taken responsibility for his death over text message.
Although she can not leave the state of MA, apply for a passport or contact members of the Roy family.
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.More news: Steve Scalise's shooter had a list with names of other congress members
Carter opted against a jury trial, leaving her fate in Moniz's hands.
Prosecutor Maryclare Flynn said that Carter "used Conrad as a pawn in a sick game of life and death for attention".
Carter also told Ms Boardman she was anxious about what investigators would find on Roy's phone. "You can't keep living this way", Michelle allegedly said in another message.
This led him to contact Carter, who reportedly encouraged him to get back into his truck.
"She did not issue a simple additional instruction: Get out of the truck."
Moniz said Carter's initial text messages pressuring Roy to kill himself were not enough to find her guilty.
Prosecutors say the then-17-year-old Carter badgered Roy to act on his suicidal thoughts. The Massachusetts woman is charged with involuntary manslaughter in the 2014 suicide of her boyfriend, Conrad Roy III.
"This was a suicide-a sad and tragic suicide, but not a homicide". Her sentencing is set for August.More news: Putin warns new sanctions will 'complicate' ties between Russia and US
And all the while, Carter kept goading him to do what he had said so many times he would do.
Attorneys for the defense said both teens had a history of depression, and Roy had previously attempted to take his life by overdosing on acetaminophen.
Prosecutors had alleged Carter, then 17, encouraged Roy in text messages. "This conviction exceeds the limits of our criminal laws and violates free speech protections guaranteed by the MA and US constitutions", ACLU of MA legal director Matthew Segal told CBS News Boston.
The northeastern state of MA, unlike other U.S. states, has no law against encouraging someone to commit suicide.
Healy said the case shows that "seemingly remote and distant communications will not insulate" perpetrators from culpability.
Medwed said the judge could consider Carter "morally blameworthy", but "moral blame doesn't always equal legal accountability".More news: Rangers' expansion draft protected list released with no surprises
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