Stephen Nichols admitted he allowed the mum of his baby to die. Now he gets to walk free.

After 23-year-old mother and aspiring model Rhonda Casto fell to her death from a cliff eight years ago, her family cried murder.

As the only person with her at the time – and as one of the few people who stood to benefit from her $1 million life insurance policy – investigators set their sights on Casto’s boyfriend and father of her child, Stephen Wagner Nichols.

But this week the 42-year-old has walked free.

Nichols was due to face trial in 2018, but in a stunning move prosecutors in the US state of Oregon have dropped the murder charge against him, as part of a deal that saw Nichols instead plead guilty to criminally negligent homicide and coercion.

In basic terms, criminal negligence applies when an individual “allowed” the victim to die.

He was sentenced to 19 months for each charge as well as three years’ of probation, but with time already served he will not be required to spend any more time behind bars, reports local news outlet, The Bend Bulletin.

Casto, who had a child with Nichols during their 3.5 year relationship, died after plunging more than 30 metres off a cliff while hiking along the scenic Eagle Creek Trail in March 2009.

Investigators initially ruled her death accidental, but in April 2014, a grand jury ruled to indict Nichols for murder. He was then arrested in February 2015, after he returned from China with his daughter.

According to The Bend Bulletin, Nichols had increased Casto's life insurance policy several months prior to her death and attempted to claim it afterward, before he eventually agreed to split the benefit with her mother, Julia Simmons, and his daughter's estate.

Nichols has maintained his innocence throughout the investigation, and his lawyer this week praised the plea deal as an "amazing outcome" according to People.

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Simmons doesn't share that enthusiasm. The grieving mother told People she believes Nichols killed her daughter because she was preparing to leave him and take their little girl with her.

“She was going to move out, but she was trying to get up the courage to do it. She was disillusioned about Steve," she said.

Describing her daughter as "precocious, loud and funny", Simmons said, "She was 23 and in the prime of her life. She had everything going for her.”

The sentencing decision has left Simmons "numb".

“This is so unfair — this is the justice system being stupid,” she said. “I am just disgusted with the outcome.”