A former Dallas officer who says she fatally shot an unarmed black man in his own home after mistaking his apartment for her own made a series of "unreasonable errors" and worried more about how the incident would affect her than about the man she shot, a prosecutor said in opening statements of her murder trial Monday.

Former Dallas police officer Amber Guyger seen Monday, September 23, 2019. AP

Amber Guyger, who is white, is standing trial for the killing of 26-year-old Botham Jean last year.

Guyger was returning to the Dallas apartment complex where she lived in an apartment on a different floor from Jean after working a 13-and-a-half-hour shift September 6, 2018, a prosecutor said. Guyger was off-duty but still in uniform when the shooting happened. She told investigators that she parked on the fourth floor of her apartment complex's garage — rather than the third floor, where she lived — and found the apartment's door ajar.

Jean, an accountant from the Caribbean nation of St. Lucia, "was doing no harm to anyone, which was his way," Dallas County Assistant District Attorney Jason Hermus said in an opening statement. He noted that Jean was in his living room eating a bowl of vanilla ice cream when Guyger entered the apartment, which was one floor directly above her apartment.

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Hermus said Guyger called 911 after shooting Jean, but on the call appeared to be more worried about herself than about the fatally wounded man. On the call, Guyger can be heard saying she shot a man because she thought he was in her apartment, and repeatedly says she's going to lose her job.

"She is as concerned or more concerned about how this is going to concern her than this poor guy on the floor next to her," Hermus said.

The case has attracted intense national scrutiny for its strange circumstances and as one in a chain of shootings of black men by white police officers. Guyger was fired after Jean's death.

Lawyers for Guyger argue she fired in self-defense based on the mistaken belief that she was in her own apartment and Jean was a burglar.

But speaking Monday, Hermus said Guyger missed obvious signs that the apartment was not hers. Holding up a red mat Jean kept outside his door, he said Guyger didn't have a mat in front of her apartment. He also said Guyger kept a large table in her home that would have been immediately visible from the front door, which Jean did not have.

"She fails to recognize all of these things that she's aware of and that failure cost [Jean] his life," Hermus said.

Amber Guyger, right, and Botham Jean. AP

A defense attorney argued that Guyger made a mistake he called awful and tragic, but innocent. Robert Rogers said the apartment complex was "confusing" and that it was not unusual for residents to mistakenly try and enter others' units. He said Guyger was exhausted and on "auto pilot" after a long shift at work and didn't notice the apartment was not hers until after she opened fire, fearing for her life because Jean was coming towards her.

"It starts to dawn on her as she approached Mr. Jean's body what a horrible, horrible mistake she has just made," Rogers said.

Rogers said Guyger immediately called for EMS and only left the apartment to show responding officers where it was located. He said Guyger, who has pleaded not guilty, would take the stand in her own defense.

The day of the shooting, Guyger had been exchanging sexually explicit text messages with her Dallas Police Department partner, Martin Rivera, with whom she had been romantically involved for about a year, according to prosecutors. During pretrial proceedings, prosecutors and defense attorneys sparred over whether those messages should be entered into evidence.

Guyger had been on the phone with Rivera minutes before she fatally shot Jean and texted him shortly afterward, Hermus said. Those texts were later deleted from Guyger's phone, he said.

Taking the stand Monday, Rivera said he began a romantic relationship with Guyger in 2017. He said nothing seemed out of the ordinary about Guyger the day of the shooting. The pair talked on the phone before the shooting about burglary suspects they arrested earlier that day, but Rivera couldn't recall what else they talked about. He said their call lasted about 16 minutes.

Rivera admitted that he deleted text messages he exchanged with Guyger on the day of the shooting from his phone.

Three days after the shooting, Guyger was arrested for manslaughter. She was subsequently fired from the Dallas Police Department and charged by a grand jury.

The trial jury will decide whether the 31-year-old committed murder, a lesser offense such as manslaughter or criminally negligent homicide or no crime at all.

The outcome may hang on whether the jury believes that Guyger's mistake was reasonable, according to legal experts.