Assistant Public Defender Dennis Burke said Peterson, 20, made the decision himself to take a plea agreement, despite ongoing preparations for a trial that was to take place in January.

"He's had a rough go of it in the jail," Burke said, including an attempted suicide in September, and is hoping that a change to prison will be better.

Also, Burke said, Peterson hated what he put his parents through at a wrenching preliminary hearing in July and he hated what he put the Marino family through and didn't want anyone to go through a trial.

"He didn't want that for these people, and for that I have to give him credit," Burke said.

But Debbie Marino said she's relieved that Peterson, a former University of Wisconsin-Madison student, will take responsibility for her stepson's death on Jan. 20. Not having to live through a trial will be a huge relief, she said.

"For many months we've been anticipating going to trial and we thought about what it's going to be like," she said. "I never thought in a million years that he would ever come back and say he'd take responsibility for this."

Peterson, of North Grant, Minn., was arrested and charged with Marino's murder in June. Burke was prepared to concede at the trial that Peterson killed Marino, but that Peterson was guilty of a lesser charge of homicide not requiring proof of intent.

Prosecutors have tapes of Peterson speaking with his parents from jail in which he admits killing Marino as part of a robbery.

Debbie Marino said she was "pleasantly surprised" by the news of the plea agreement, which was also relayed to Lou Marino, her husband and Joel Marino's father, while he is away in Florida. She said her husband will be back in Madison tonight and they'll attend the plea hearing set for Peterson for Monday morning.

"It's the best news out of a terrible situation," she said.

If Peterson pleads guilty to first-degree intentional homicide as expected, he would face a mandatory life prison sentence. Dane County Circuit Judge James Martin would then decide, with the help of a pre-sentence report, how long Peterson would have to stay in prison before he is first eligible to be released on extended supervision. Under state law, the minimum period before eligibility is 20 years.

Burke said Assistant District Attorney Corey Stephan agreed to ask that Peterson be made eligible for release in no more than 40 years.

Still, Marino said, there are questions about why Peterson decided to go to her stepson's house at all, and once there, decided to stab him. Those are the kinds of things she would like to hear from Peterson, to help her family make sense of what happened, she said.

"It's just a terrible waste," she said, "for the Petersons and for us."