US mother breaks down at New York killer nanny trial
Leo and Lucia Krim (Facebook)

The devastated mother of two young children killed by their New York nanny broke down on the stand Friday, yelling that her former employee was "evil" and a "liar."

Yoselyn Ortega is on trial for murdering Lucia, six, and Leo, two, with a kitchen knife in the bathroom of their Upper West Side apartment on October 25, 2012. Ortega, 55, does not deny the killings, but is pleading insanity.

It was a case that terrified working parents all over the world and went on to inspire a best-selling novel that explored the relationship between a working mother and the woman she hires to look after her young children.

Marina Krim, 41, broke down in tears and anger, leaving the stand in the midst of being cross-examined after becoming increasingly enraged by the questions.

"Because she is evil," she screamed as defense lawyer Valerie Van Leer-Greenberg tried to resume questioning. "You are liars," she shouted. "You are evil," she added as she left the courtroom for a brief break through the witness room door.

After she returned to court, the judge waived objections from the defense to allow the prosecuting lawyer to talk Krim through an incident she wanted to tell the court.

Stifling her sobs, Krim recounted the pain of suffering a miscarriage shortly before the killings and of Ortega's response that particularly troubled her.

Krim said when she first told the defendant that she was pregnant, Ortega "gave me this giant hug, and she said, 'I am so happy, I love you, I love you.'"

But when she later told her about the miscarriage, "she did not give me a hug, she gave me no emotion at all, she actually looked mad at me that I had a miscarriage."

Krim then screamed: "She is a narcissist, it's all about money, it's all about money!"

When the prosecution tried to move on, Krim left the room shouting and did not return, leading the prosecuting lawyers to call another witness.