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Differing portraits painted of N.H. murder defendant

The jury will start deliberating a first-degree murder charge against Seth Mazzaglia (right) after receiving instructions from the judge on Thursday.

Jim Cole/Associated Press

The jury will start deliberating a first-degree murder charge against Seth Mazzaglia (right) after receiving instructions from the judge on Thursday.

DOVER. N.H. — The man behind the 2012 murder of a 19-year-old University of New Hampshire sophomore is a cold-blooded killer who was overtaken by “the darkness,” a state prosecutor said Wednesday in closing arguments.

“He was not in control of Lizzi, and he couldn’t stand it. And he killed her for it,” Assistant Attorney General Geoffrey Ward said, describing how 31-year-old Seth Mazzaglia allegedly strangled Elizabeth “Lizzi” Marriott of Westborough, Mass., and raped her lifeless body after she rejected his advances.

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Mazzaglia’s attorney, Joachim Barth, attacked the credibility of the prosecution’s star witness, the defendant’s former girlfriend. He said Kathryn McDonough, who spent 10 days on the stand, was a sociopath, a liar, and a manipulator who changed her story in exchange for a shorter prison sentence.

“She needs this more than anything, and you don’t think she’s willing to lie?” he said to the jury.

McDonough, who was given a plea deal for her testimony, has told the court that Mazzaglia pressured her to lure Marriott to the couple’s apartment as a sexual offering to him, and that she watched him strangle and rape Marriott. The defense has argued that McDonough, 20, killed Marriott during rough sex.

The trial, which included 19 days of testimony, featured strange and lurid testimony about the young couple, delving into everything from their sex life to their belief in alternate personas, dragons, and tarot cards.

Much of the case hinges on power dynamics, with both sides debating who was the dominant figure in the relationship.

In the tense courtroom Wednesday, Ward said the defense’s focus on undermining McDonough distracted from “the truth of the defendant.” He portrayed Mazzaglia as a controlling alpha male who “used [McDonough’s] insecurities, her vulnerabilities, and her teenage angst to draw her in.”

Mazzaglia was used to getting his way, Ward said, so Marriott’s refusal of sex enraged him. Mazzaglia put on gloves, selected a rope, and strangled her to death, Ward said, stressing to the jury his deliberate actions constitute premeditation.

“This defendant made clear if his sexual desires were not fulfilled, ‘the darkness’ would get the better of him,” Ward said.

His last words to jurors asked them to hold Mazzaglia responsible.

“Send him the message that you . . . know the truth about him, that he is a cold-blooded killer, and deliver him the justice that he has tried, in so many ways, for so many times, to avoid,” Ward said.

Barth relentlessly attacked McDonough’s credibility.

“What we have is Kat McDonough being the manipulator of facts,” Barth said.

He said her narrative of abuse and submission was meant to distance herself from blame for the murder. He said she manipulated Mazzaglia, who just wanted to help her escape a home life she told him was abusive.

“The real issue is how erratic, how unpredictable and how unreliable she is,” he said.

Barth asked jurors to consider the probability Mazzaglia would rape Marriott after he had already called friends to come to the apartment.

“Does that make sense to you ?” he said.

Mazzaglia allegedly murdered Marriott on Oct. 9, 2012, at the Dover apartment he shared with McDonough. McDonough testified that she helped Mazzaglia dump Marriott’s body in a suitcase off Peirce Island. Marriott’s body has never been found.

McDonough is serving a 1½-to-three-year sentence for conspiracy, hindering prosecution, and witness tampering.

Claire McNeill can be reached at claire.mcneill@globe.com.

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