Gruesome Testimony in Kinky Sex Trial

Twenty-year-old Kat McDonough told a stunned jury she was in a sick relationship with karate instructor Seth Mazzaglia.  

He called himself "Dark Heart" and called her "Skarlett." She says he was obsessed with kinky sex.

Reporter Kim Haas has been following the story. She told INSIDE EDITION, “He was expressing from the start of the relationship that she must bring another woman into it.”

He sent his girlfriend a disturbing text saying: "Choose a friend. Any of yours will do. And you offer her to me."

She says she lured a co-worker at a Target store, 19-year-old University of New Hampshire student Lizzi Mariott to his apartment. But when Mariott refused to have sex, Mazzaglia raped and murdered her while McDonough watched.

In court, McDonough  said, “He pulled the rope over her neck and strangled her.”

A lawyer asked, “Did you try to stop the defendant?”

“No. He was my world. I didn't have anyone else,” said McDonough.

At first, McDonough told police Mariott died during rough sex. Then she made a deal with prosecutors to testify against her boyfriend in return for immunity.  

Investigators even taped a jailhouse phone call in which Mazzaglia tried to persuade her to marry him so she couldn't be forced to testify against him.

In a call, he said, “I've been trying to figure out what the policy of marriage while someone's in jail is.”

McDonough said, “I know you can do it. Some jails require me to write a letter why I would actually want to marry somebody.”

He replied, “I would hope, 'Madly in love' would count.”

“Yes,” she said.

Mazzaglia claims his girlfriend is the real killer and he only helped her dispose of the body. But the jury wasn't buying it.

The judge asked, “First degree murder by strangulation, what is you verdict? Do you find the defendant guilty or not guilty?”

The jury responded, “Guilty!”

There was more drama at the sentencing hearing as the victim's mom confronted the killer.

She said, “I'm Lizzi's mother and I want you to know unequivocally that I hate you. You are a cowardly and despicable person.”

And in a heartbreaking moment, Lizzi's dad showed his grief by parading a life-sized photo of his daughter around the silent courtroom.

The judge gave Mazzaglia the maximum possible sentence, saying, “You are sentenced to the New Hampshire State Prison for life and shall not be eligible for parole.”

As the killer was led away to prison his victim's dad said the life sentence is small comfort, saying, “It's just a legal proceeding. The loss of Lizzi is what I don't know how I’m going to get past.”