Amber Guyger told jurors that the shooting "is not about hate, it's about being scared."

A former Dallas police officer charged with murdering an unarmed neighbour when she walked into his apartment thinking it was her own, wept as she testified in her own defence on Friday, saying she asks God daily for forgiveness.

Amber Guyger, 31, was coming off a 13-1/2 hour shift when she walked into the central Dallas apartment of Botham Jean, a 26-year-old black PwC accountant, and shot him as he ate ice cream, thinking he was a burglar in her apartment one floor lower.

"I wish he was the one with the gun and killed me. I never wanted to take an innocent person's life," Guyger testified, adding that at the time she had believed her life was in danger. "I'm so sorry, I'm so sorry. I have to live with this every single day."

None of the 12 jurors betrayed emotion as Guyger wept.

Guyger, who is white, said she mistakenly thought she had parked on her own level in her apartment building's parking garage after her shift.

She told jurors that the shooting "is not about hate, it's about being scared."

The judge briefly halted the proceedings to allow Guyger to compose herself.

It is relatively rare for criminal defendants to testify in their own defence at trial since it subjects them to cross-examination by prosecutors who will attempt to poke holes in their story.

"When you shot him twice, you intended to kill him, didn't you?" Assistant District Attorney Jason Hermus asked Guyger.

"I did," she responded, in a calm voice.

Hermus sharply questioned Guyger about why she performed virtually no emergency procedures to try to save Jean, and why in her panicked call to 911 just after the shooting she never said she thought Jean had a weapon or that she had been scared for her life. The officer said she was in panic and that her mind was racing at that time.

The prosecutor also pressed Guyger on why she simply did not choose to not enter the apartment she thought was her own if she felt her life was in danger. The officer acknowledged that she could have done that - and that Jean would still be alive today if she had.

Guyger faces life in prison if found guilty of murder.

The jury could also find her not guilty, or guilty of the lesser crimes of manslaughter or criminally negligent homicide. Manslaughter carries a maximum sentence of 20 years in prison, while she would face a total of two years in prison for criminally negligent homicide.

The shooting, one of a series of high-profile killings of unarmed black men and teens by white U.S. police officers, sparked street protests, particularly after prosecutors initially moved to charge Guyger with manslaughter, a charge for killing without malice.

In contrast to cases like the killings of Michael Brown in Missouri and Philando Castile in Minnesota, Guyger shot Jean while she was off duty, rather than while responding to a reported crime.

During the trial, Guyger's defence attorney, Robert Rogers, said she was "on autopilot" after a long work day on Sept. 6, 2018, when she mistakenly parked on the wrong floor in the garage and was able to enter Jean's apartment because he had left the door slightly ajar.

Hermus told the jury of four men and eight women that Guyger missed blatant clues that she was not in her own apartment - including the smell of marijuana smoke - because she was distracted after a 16-minute phone conversation with her former police partner during her commute. Guyger testified that the call was in relation to work.