A Woburn High student who witnessed the stabbing death of his friend’s mom over video chat described the attack in horrifying detail in Middlesex Superior Court yesterday, saying 40-year-old Christopher Piantedosi was “wide-eyed” and clenching his jaw as he repeatedly stabbed his longtime girlfriend with a kitchen knife in her Burlington home.

“She was saying ‘No, Chris, stop, I love you,’ ” said Michael Edwards, 16, testifying at Piantedosi’s murder trial in the May 3, 2012, killing of Kristen Pulisciano.

Edwards, who had been chatting with Piantedosi and Pulisciano’s then-15-year-old daughter in the moments before the attack, said Piantedosi violently shook the crying, 38-year-old mother of two in the girl’s bedroom, saying, “You’ve got to die.”

Piantedosi of Methuen looked at Edwards directly during the testimony on the first day of the trial, closing his eyes momentarily and letting out short sighs.

A badly bloodied Pulisciano was found on the floor with a “large knife protruding from her neck,” Burlington police officer Joseph Papsedero said.

Defense attorney Michael Collora said he is “not going to dispute the evidence.”

“We are going to suggest … he did not have the mental capacity and intent to commit this murder,” Collora said.