WAUKESHA COUNTY, Wis. (CBS 58) -- A tragic family murder saga will end Thursday, March 5, in a Waukesha County courtroom. But the emotional pain that was caused will last much longer.

Nikita Casap will be sentenced for killing his mother and stepfather.

Now, new documents try to explain why he did what he did.

Casap must be sentenced to life in prison after pleading guilty. What's at stake is whether he's eligible for parole after serving 20 years.

The documents make the case on either side and offer a look into his state of mind, and the damage he's caused.

A letter to the judge, written by Nikita Casap's aunt, reads, "The loss is permanent. The pain is permanent."

"The act that took Don and Tatiana's lives was cruel, selfish, heartless, and unforgiveable. It did not end with them. It shattered a family that was whole."

The victim impact statement details the family's struggles since Casap murdered his mother and stepfather in their Waukesha home last February.

"Grief lives in our home now. It has changed the air in our house. It has changed the way we speak to each other. It has changed the sound of laughter. It has changed us."

The now-18-year-old Casap pleaded guilty in January to two homicide charges. Several other charges were dismissed.

Also filed was an evaluation from the State Public Defenders Office.

It says Casap "was under the influence and psychological control of a highly manipulative individual."

In arguing for leniency, the report details Casap's birth and early years in Moldova before he immigrated to the us with his mother and stepfather.

Casap struggled to make friends and instead built an "increased reliance on the internet" that "affected his emotional well-being."

He soon became exposed to "unhealthy and harmful influences."

"By 2022," it reads, "Nikita had begun identifying with neo-Nazi ideology."

When his mother found out, she cut his access to his phone and computer.

In 2023, the FBI interviewed Casap at home "after becoming aware of concerning online search activities."

Gradually, one forum user "introduced more extreme ideological content."

The report also describes a transformation after the murders and his arrest, saying Casap has since showed remorse and self-reflection.

But his aunt is asking for "severe consequences."

"Nothing will bring Don and Tatiana back. Nothing will restore what was stolen. But accountability matters."

Nikita Casap is scheduled to be sentenced at 1:30 p.m. Thursday in the Waukesha County Courthouse.