A Dallas police officer started to cry and shake Friday as she began to testify about the night she killed a neighbor in his own home, which she has said she mistook for her own unit one floor below.

Amber Guyger broke down while recalling approaching her neighbor Botham Jean’s door, leading the judge to call for a brief break so she could compose herself.

Guyger, who is charged with murder in the killing last September, was the first witness that her lawyers called in the high-profile case. She told jurors about how she grew up in a small house in the Dallas suburb of Arlington, how she played in the school band and how she aspired to become a police officer.

“I just wanted to help people, and that was the one career that I thought I could help people in,” said Guyger, who was fired from the police force after the shooting.

Guyger’s testimony marked the first time the public has heard directly from the 31-year-old since Jean’s killing last September.

Guyger, who is white, was off duty but in uniform when she shot Jean, a 26-year-old native of St Lucia who was black and worked as an accountant in Dallas.

The basic facts of the unusual shooting are not in dispute. Guyger walked up to Jean’s apartment – which was on the fourth floor, directly above hers on the third – and found the door unlocked. She was off duty but still dressed in her police uniform after a long shift when she shot Jean with her service weapon. Guyger was later arrested, fired and charged with murder.

“I hate that I have to live with this every single day of my life and I ask God for forgiveness, and I hate myself every single day,” Guyger told the packed courtroom.

She said she wished “he was the one with the gun” and had killed her. “I never wanted to take an innocent person’s life. And I’m so sorry. This is not about hate. It’s about being scared that night,” she said.

Guyger’s attorneys say she fired in self-defense after mistaking Jean for a burglar. Her attorneys also say the identical physical appearance of the complex from floor to floor frequently led to tenants parking on the wrong floor or trying to enter the wrong apartment.

Guyger told the jury that when she put her key in what she thought was her apartment door’s lock, the door opened because it hadn’t been fully closed.

Fearing it was a break-in, she drew her service weapon and stepped inside, she said.

Seeing someone inside, Guyger said she said: “Let me see your hands! Let me see your hands.” But she said she couldn’t see the person’s hands and he began coming toward her at a “fast-paced” walk. She said he yelled “Hey, hey, hey!” right before she opened fire.

Guyger told the court she intended to kill him when she pulled the trigger, which she said is what she had been trained to do.

“I was scared he was going to kill me,” she said under questioning by her lawyers, who called her as their first witness.

When prosecutors asked why she didn’t back away and radio for help once she suspected a break-in, Guyger said that entering the apartment “was the only option that went through my head”.

Prosecutors have questioned how Guyger could have missed numerous signs that she was in the wrong place, and suggested she was distracted by sexually explicit phone messages with her police partner. Prosecutors also say Jean was no threat to Guyger, noting that he was in his living room eating a bowl of ice-cream when she entered his apartment.

In a frantic 911 call played in court earlier this week, Guyger says “I thought it was my apartment” nearly 20 times. The shooting attracted intense national scrutiny for the strange circumstances and because it was one in a chain of shootings of unarmed black men by white police officers.

The trial began Monday.