Fox News Senior Judicial Analyst Judge Andrew Napolitano said Tuesday that former Fort Worth police officer Aaron Dean will have to prove to the jury that he had a "justifiable fear in his mind of physical harm" after killing 28-year-old Atatiana Jefferson in her mother's home while responding to a call on Saturday.

Appearing on "America's Newsroom" with anchor Bill Hemmer, Napolitano explained that the claim would require Dean to demonstrate to the jury that he had that fear of harm to himself and that the "only way to address that fear was the use of physical force."

"This is a subjective test, meaning: 'Did the fear exist in his mind?' Not an objective test. The question is not, 'Was the fear rational?' The question is, 'Did the fear exist?'" he asked.

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Napolitano cited another recent case in Texas in which Amber Guyger, a former Dallas police officer convicted of murder in the shooting that killed neighbor Botham Jean, was sentenced by a jury to 10 years behind bars.

"Her defense was that it was an honest mistake," he said. "But, she was ultimately not able to prove that defense."

Dean was arrested hours after resigning from the Fort Worth Police Department. In a statement, the Department said that officers saw someone near a window inside the home and that one of them drew his duty weapon and fired after "perceiving a threat."

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The body camera video released shows two officers searching the home from the outside with flashlights. Then one officer shouts, "Put your hands up, show me your hands." A single shot is then fired through a window.

In the video, the officer does not identify himself as police.

Fort Worth Mayor Betsy Price said that the killing of Jefferson was unjustified.

On Monday, Price told reporters: "I'm so sorry. On behalf of the entire city of Fort Worth, I'm sorry. To Atatiana's family, it's unacceptable. There's nothing can justify what happened on Saturday morning. Nothing."

"At the time the Chief made that statement, he went through a litany of things which -- according to the Chief -- officers of Fort Worth are trained to do which he did not do," said Napolitano. "Which...would have made it very difficult for them to defend him."

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Napolitano told Hemmer that in most states the prosecution must disprove the affirmative defense. However, in Texas the burden of proof is reversed--it's on the defendant.

"It's much higher and a very difficult burden to meet," he added.

Fox News’ Greg Norman, Mike Arroyo, Louis Casiano, Melissa Leon, and The Associated Press contributed to this report.