At nanny murder trial, mother recounts 'horror' of seeing her 2 kids dead: 'She killed my best friends' Yoselyn Ortega is accused of killing a 6-year-old and a 2-year-old.

 -- The mother of two young children stabbed to death in their New York City home, allegedly by their nanny, in court today recounted the moment she came home to find her son and daughter covered in blood in the bathtub.

“She killed my best friends," Marina Krim said of her former nanny, Yoselyn Ortega.

"You’re disgusting," Krim said to Ortega as she left the stand.

Ortega is on trial for allegedly killing 6-year-old Lulu Krim and 2-year-old Leo Krim at the Krim family's idyllic Upper West Side apartment on Oct. 25, 2012, in a crime that shocked parents in the largest U.S. city.

Ortega, now 55, was charged with two counts each of first-degree murder and second-degree murder and is pleading not guilty by reason of insanity.

Prosecutors say Ortega knew what she was doing and "intentionally and savagely slaughtered" the two children in her care because she resented the children's mother. Prosecutors say, despite what defense attorney claim, there's "no credible evidence" to establish any mental disease or defect.

6-year-old fought back, prosecutors say

Ortega, who worked for the children's parents, Marina and Kevin Krim, for two years, took knives from the family's kitchen and repeatedly slashed and stabbed the children in the bathroom before carefully laying their bodies down, Assistant District Attorney Courtney Groves said in today's opening statements.

Groves alleged Ortega slit their throats to make sure neither could scream for help.

Leo was too small to struggle, but Lulu, 6, "knew what was happening" and fought back, according to Groves, citing cuts to hand and arms are consistent with defense. Lulu had about 30 different stab and slashes, Groves said.

Mother comes home to children's 'bloodied, lifeless bodies'

Marina Krim came home after taking her then-3-year-old daughter, Nessie, to a swim class, Groves said. Walking toward the bathroom was a "total horror movie," Marina Krim said on the stand today as she testified for the prosecution.

Marina Krim opened the bathroom door and found "the bloodied, lifeless bodies of her 6-year-old daughter and 2-year-old son ... their eyes open, covered in blood," Groves said.

Ortega, who was standing in the bathroom, then stabbed herself in the neck, according to Groves.

Marina Krim grabbed Nessie and ran out of the apartment and the doorman called 911, prosecutors said.

Marina Krim said that moment she was screaming uncontrollably and saying, “I just saw my kids dead."

Marina Krim said she banged her head as hard as she could on marble pillars “to wake up from this nightmare.”

“I was destroyed,” Marina Krim said on the stand.

The children's neck wounds were so severe that first responders thought they may have been decapitated, Groves said.

Leo was so covered in blood, the officer who removed him from the bathtub feared Leo might slip from his grasp, Groves said.

A sheet was thrown over Marina Krim's head, and from underneath the sheet, she could see her children being rolled out on the gurneys, she said in court.

Kevin Krim, who had been flying back from a business trip in California that day, was met by New York City police homicide detectives at the airport, Groves said.

'You will know a diseased mind when you see it'

Oretga has no prior arrest record and is a college graduate who "has worked since she could walk and talk," defense attorney Valerie Van Leer-Greenberg said in opening statements.

On the day of the children's deaths, and before that day, Ortega suffered from a "chronic mental illness," Van Leer-Greenberg said.

"She has a corroborated history of hearing voices and disassociating from reality since the age of 16," and has suffered from depression, psychotic thinking and hallucinations, Van Leer-Greenberg said.

The evidence presented at trial will show the condition of Ortega's mind at the time of the children's deaths, the attorney said.

"You will know a diseased mind when you see it," Van Leer-Greenberg said. "Do listen to the medical experts."

The "lack of motive in this case is the hallmark of a mentally ill offender," she added. The jury will have to determine if she "had an ability to understand, perceive and confront her illness" that day.

Prosecutors allege Ortega resented Marina Krim

Groves said evidence suggests Ortega resented Marina Krim for being the mother she couldn’t be and for providing for her children what she couldn’t.

Ortega had left her son in the Dominican Republic to be raised by her sister and her son came to the U.S. just months before the Krim children's deaths to finish high school, Groves said

As soon as her son came, Ortega’s stress and anxiety mounted, and the financial strain of her son’s private school tuition “weighed heavily” on Ortega, Groves said.

Ortega made a psychologist appointment days before the killings. That meeting was about Ortega's stress, anxiety, feelings of failure and relationship with her son, Groves said, adding it was the only contact the defendant had with any mental health professional in 30 years.

Groves claimed Marina Krim went out of her way to try to help Ortega make more money, by doing housecleaning at the Krims as well as recommending Ortega to other people.

But Groves said Ortega was "enraged" by that internally, and following the deaths, Ortega in interviews blamed Marina Krim for problems in her own life.

But Van Leer-Greenberg said the rantings of a "delirious medicated person" shouldn't be relied upon to determine a motive.

'We're going to focus on the positive'

Since Lulu and Leo's deaths, the Krims have had two more children, Felix and Linus.

They also founded the Lulu & Leo Fund, an organization that offers parents and schools a creativity curriculum based on 10 principles of creativity "that can help anyone thrive and build resiliency in all facets of their lives," according to its website.

The initiative "empowers children and adults with the skills and confidence they need to have resilience in the face of hardship and to thrive in an ever-changing world," Kevin Krim wrote in an essay last year. His essay appeared in Option B, a website started by Facebook COO Sheryl Sandberg, whose husband died unexpectedly in 2015.

"Marina and I shared the creative impulse to do something constructive in the face of the destructive effects of violence," Kevin Krim wrote. "We started the Lulu & Leo Fund in those early days to honor their creative, too-brief lives."

In a video message last month, Kevin Krim said, "This trial will be very hard for us."

But, Marina Krim added, referring to the Lulu & Leo Fund, "We're going to focus on the positive and the goodness that's come out of all this."

ABC News' Katie Kindelan contributed to this report.

Sheryl Sandberg is a member of the board of The Walt Disney Company, the parent company of ABC News.