DOVER, N.H. — The man charged with killing a Massachusetts coed became enraged when she denied his sexual advances, sneaked up behind her, and choked her with a rope before raping her, prosecutors said at the opening of his trial on Wednesday.

Seth Mazzaglia, 31, is charged with first-degree murder in the death of Elizabeth ‘‘Lizzi’’ Marriott. He has said she died during rough consensual sex and his lawyers argued that his girlfriend at the time was responsible for Marriott’s death.

Marriott, of Westborough, Mass., was a 19-year-old student at the University of New Hampshire who was lured to Mazzaglia’s apartment by Kathryn McDonough, his girlfriend, on Oct. 9, 2012.


During opening statements, the prosecution painted a lurid picture of Mazzaglia as sexually dominant and said he took Marriott by force when she would not come to him willingly. Assistant Attorney General Peter Hinckley said Mazzaglia asked Marriott for sex after playing a game of strip poker at the apartment he shared with McDonough.

‘‘Lizzi said no,’’ Hinckley said.

Mazzaglia then said he was going to have sex with McDonough while Marriott watched, Hinckley said.

‘‘She rejected the master again, in his home, where he got whatever he wanted sexually,’’ Hinckley said. ‘‘So he took what Lizzi denied him.’’

Hinckley said Mazzaglia put on gloves, walked to the back of the couch where the two young women were watching a movie and slipped a rope around Marriott’s neck.

‘‘He yanked back hard and Lizzi never had a chance,’’ Hinckley said. ‘‘The defendant struck quick, without warning and with absolutely no mercy. Lizzi had time to let out a quick, terrifying cry, but had time for little else. He choked her for minutes until she could no longer deny him.’’

Then Mazzaglia raped her as McDonough watched, Hinckley said.

Marriott’s body was thrown into the Piscataqua River and has not been recovered.


Marriott’s parents sat in the front row of the courtroom, her mother softly weeping at times.

Defense lawyers blamed McDonough, saying Marriott died when McDonough trussed her with a harness and collar and suffocated her during a rough sex act.

The defense team cast her as the sexual manipulator in her relationship with Mazzaglia and said she lied and will testify against him to get a more lenient sentence.

McDonough pleaded guilty in July to hindering the prosecution, conspiracy, and tampering with a witness and was sentenced to 1½ to 3 years in prison. Hinckley acknowledged that McDonough could have received a harsher penalty.

‘‘But for all McDonough did, she did not rape Lizzi and she did not kill Lizzi,’’ he said.

Hinckley said Mazzaglia sent McDonough a three-page, handwritten note detailing alibis and a plot to frame two friends for killing Marriott. He told McDonough to ‘‘rip and flush’’ the note. She ripped it up, but kept the shreds, which Hinckley said investigators reconstructed.

Strafford County Superior Court Judge Steven Houran denied a prosecution motion Wednesday for a mistrial.