Prosectors argued in a Dallas courtroom that a white former Texas police officer committed a “commando-style” murder of a black neighbour after entering his apartment by mistake.

They dismissed as “garbage” her argument that she had thought she was in her own apartment and shooting an intruder, when in fact she was the intruder, mistakenly entering another apartment.

Amber Guyger was justified in shooting black man, Dallas ex-police chief says Read more

Amber Guyger is on trial for murder in the death of Botham Jean, a 26-year-old accountant who was settling down to watch television on the evening of 6 September last year when she shot him dead. Guyger has claimed that after returning home after work, she failed to realise she was on the floor above the one where her own apartment was and assumed Jean was an intruder.

The jury was deliberating on Monday amid heightened tensions in the city. The shooting sparked protests, with civil rights activists arguing that it was an example of injustice in a metropolitan area where police have long been accused of racial bias and excessive force.

“A guilty verdict in this case does not mean you hate police,” prosecutor Jason Fine told jurors. “This has to do with that defendant making unreasonable decisions that put her in that seat and Bo in the ground.”

Guyger’s attorneys asserted that her behaviour was reasonable because she thought Jean was a burglar who represented a threat to her life, entitling her to shoot in self-defence in a split-second decision that was a tragic error.

But prosecutors contended that Guyger was ready and willing to use her weapon from the moment she thought someone had broken into her apartment in central Dallas. “She made that decision outside the door [that] she was just going to go in commando-style,” Fine said during closing statements.

Guyger, 31, was an officer with the Dallas police department for more than four years. She was fired after being arrested for manslaughter; the charge was later increased to murder.

She wept on the witness stand last week as she claimed that she had ordered the St Lucia-born man to show his hands then shot him because she was scared he was going to kill her. “I hate myself every single day,” she said. “I wish he was the one with the gun [who] had killed me … I never wanted to take an innocent person’s life.”

Fine, though, told the jury on Monday that “most of what she said was garbage”. He claimed that Jean was on his couch, set to watch television and eating a bowl of ice cream, when Guyger burst in, startled him and shot him in the chest.

“He starts to get up like a normal, reasonable person who has somebody busting into his home, and before he can even get up he is shot dead in his own home,” Fine said, ridiculing the idea that the unarmed man represented an immediate threat to Guyger’s life. “He’s not going to throw a spoon at her and kill her,” he said.

Jason Hermus, another prosecutor, held a large photograph of a smiling Jean, as he told the jury: “Killing this man was unnecessary and unreasonable from start to finish … this is not one of those things where you can just say ‘whoops, too bad for you, buddy, you’re buried six feet under, I’m going to get on with my life’.”

The sides differed on Jean’s exact position when he was shot and whether Guyger ought to have known she was in the wrong location. Prosecutors said it was ludicrous that a trained officer should claim to have missed visual indicators, most notably the bright red floor mat in front of Jean’s door.

The defence said Guyger, who was still in uniform, was tired after a long shift and that interviews with other residents of the complex indicate that going to the wrong floor is a common occurrence. “She made a series of horrible mistakes,” Toby Shook, one of her attorneys, said. “The law recognises that mistakes can be made.”

The prosecution said she failed to follow procedure by not taking cover and calling for back-up and questioned whether she had given verbal commands to Jean, arguing that no neighbours heard her do so.

The jury was asked to decide whether Guyger’s conduct was a reasonable “mistake of fact” and whether she could plausibly assert that she acted in self-defence under the state’s so-called “castle doctrine” because she believed – however erroneously – that she was in danger on her property.

Prosecutors, though, asserted that the doctrine did not apply. “It protects homeowners against intruders – and now all of a sudden the intruder is trying to use it against the homeowner?” Fine said.

Guyger pleaded not guilty. The state district judge Tammy Kemp gave detailed instructions to the jury and told them they could consider a verdict on the lesser charge of manslaughter if they felt Guyger acted “recklessly” rather than with intent – though Guyger testifed that she shot to kill, in line with her police training.

Testimony began on 23 September after an arduous jury selection process, with defence attorneys failing to persuade Kemp that the high-profile trial should be moved to another county because of the potential prejudicial effect of what they called “media hysteria”. Kemp did, however, sequester the jury.