Sandoval is on trial for a second time after a judge declared a mistrial due to an 11-to-1 hung jury.
She has admitted to pulling the trigger after breaking into Green's home in 2015.
She told the jury she broke into the house to gather some of their child's belongings.
The defense claimed she acted on fear and impulse following a years-long abusive relationship.
Sandoval testified in court about an incident during her lunch break at work.
"We were just arguing and he grabbed me by the hair and he tried dashing my head on the dashboard," said Sandoval.
"Once he wasn't putting his hands on me, he was always verbally abusive but I think it became more intense like verbally abusive and mentally abusive."
Lead prosecutor David Alavezos argued the crime was premeditated and Sandoval caught Green off guard as he was in the restroom.
Alavezos believes the motive may have been a new relationship.
Green's family was too devastated to be on camera but told Action News he was a great guy.
Green was a ten-year veteran of the Exeter Police Department.
Sandoval's mother, Irma, was also called to testify in court on Thursday, sharing moments she says Daniel became aggressive with her daughter.
"When Daniel arrived, when he was entering, Daniel was coming saying many bad words," said Irma.
Irma told the court she never called the police because Daniel was an officer himself.
"Because we were afraid because police officers are always on the right," Irma said.
Sandoval also testified that Green had pointed a gun at her in the past and wrote a poem threatening to kill her.
That poem in a text message partially read:
"So learn your own place, and be my b****, Or I'll leave without a trace, and leave you in a ditch."
The prosecution is scheduled to cross-examine Sandoval next week when the trial continues.
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