< PREVIOUS SLIDE SLIDE 1 of 8 NEXT SLIDE > Fired Dallas police officer Amber Guyger becomes emotional as she testifies in her murder trial, Sept. 27, 2019, in Dallas. © Tom Fox/The Dallas Morning News via AP

The former Dallas police officer accused of killing an unarmed man in his apartment took the stand in her own defense on Friday, overcome with emotion as she was instructed to tell the jurors what happened when she opened the apartment door.

"I was scared to death," Amber Guyger testified, adding that her "heart rate just skyrocketed."

Guyger, who is white, walked into the apartment belonging to Botham Jean, who was black, on Sept. 6, 2018, allegedly believing it was hers. Guyger was still wearing her police uniform when she fired two shots at Jean, who was eating ice cream in his unlocked apartment.

Guyger was fired from the Dallas Police Department weeks after the shooting. She is charged with murder.

"I'm so sorry," she said as she wept on the stand, her voice trembling. "I never wanted to take an innocent person's life."

Guyger testified Friday that the night of the shooting she was tired from a long day at work and mistakenly parked on the wrong floor. She said the parking floors at her apartment building were not clearly marked.

She re-enacted how she reached the apartment door, with her backpack, lunchbox and police vest in her left hand, and testified that she heard the sound of someone walking inside.

When Guyger put the key into the lock that night, she said she noticed the door was "cracked open" and that putting the key into the lock forced the door open to the dark apartment. Guyger said earlier she had experienced problems getting the door to lock completely at her apartment.

Jean, an accountant for the international auditing firm PricewaterhouseCoopers, was sitting on his couch when Guyger opened the door, prosecutors said.

Guyger said she saw the silhouette of a figure so she pulled her "gun out and I yelled at him," shouting, "Let me see your hands! Let me see your hands!"

She told the jurors the figure was moving around and she could not see his hands, and that the man "was yelling, 'Hey! Hey! Hey!' in an aggressive voice."

Guyger reenacted the next moment for the jurors, holding her right hand out as if she was holding a gun. Her attorney asked why she fired, and Guyger replied, "I was scared he was gonna kill me."

After the shooting Guyger said she realized she was not in her own apartment.

"I had no idea where I was at," she testified, so she went outside to look at the apartment door.

The defense attorney played Guyger’s emotional call to 911 in which she repeatedly said she thought she was entering her apartment.

Guyger said she did a sternum rub on the victim, which is often performed by EMTs.

"I wanted him to keep breathing," she testified. "The state he was in, I knew it wasn’t good."

But during cross examination, prosecutors accused Guyger of not giving Jean undivided attention for CPR.

Guyger admitted to trying to perform a "little CPR," but she said she kept getting up to figure out where she was and tell 911. Guyger also admitted to stopping CPR to text her police partner and lover to come help. Guyger said she did not have any blood on her uniform or shoes.

During cross examination Guyger said she could have called for help on her radio or backed out of the apartment instead of reaching for her gun.

Guyger, speaking softly, admitted her intention was to kill Jean when she pulled the trigger.

Dallas County Assistant District Attorney Jason Hermus told the jury in his opening statement that Guyger fired so quickly she gave Jean "no opportunity for de-escalation, no opportunity for him to surrender."

"Bang, bang. Rapid," Hermus said.

Hermus said Guyger's apartment was directly beneath Jean's fourth-floor unit. Not only did Guyger mistakenly park on the wrong floor of the complex, she walked down a long hallway, passing 16 different apartments, but failed to realize she was not headed to her front door, Hermus said.

Hermus said Guyger appeared to be planning a rendezvous with her police partner and lover. Hermus showed the jury text messages Guyger sent her partner moments before the shooting and argued that during that communication, Guyger became distracted and confused about where she was.

"In the last 10 minutes of Bo's life, Amber Guyger made a series of unreasonable errors, and unreasonable decisions, and unreasonable choices," Hermus said.

Defense attorney Robert Rogers in his opening argument described the configuration of the apartment complex, where Guyger had lived for about two months, as "a confusing place" with floors in the parking garage and apartment doors not clearly marked.

"After this incident, the investigators interviewed and learned that 93 tenants had unintentionally parked on the wrong floor," Rogers said.

He said another 46 tenants who lived on the floors where Guyger and Jean resided had gone to the wrong apartment and placed their key in the door.

Rogers denied that Guyger was planning a rendezvous that night with her partner, calling the prosecution's assertion "speculation."

This is a developing story. Check back for more updates.