DALLAS — Botham Shem Jean analyzed risk for a living at a global auditing firm. For someone in his line of work, the evening was shaping up to be as risk-free as it gets: Alone, in his one-bedroom apartment one block from the Dallas Police Department headquarters.

Fresh from work, he had texted his sister his evening plans: Watching a football game on TV, the Eagles versus the Falcons. He texted a friend, apologizing for not going out with her the weekend before. Mr. Jean, 26, was from the island-nation of St. Lucia. He had a big smile, and was a big eater, winning a meat-lovers’ contest at Big Chef Steak House back in the Caribbean. He still had his ticket for a free meal on his next visit, his prize after eating a two-pound steak in one sitting.

Unit 1478 on the fourth floor of the South Side Flats apartment complex was an 800-square-foot bachelor pad: dishes piled up in the sink, with pancake syrup, dish soap and other belongings adding to the clutter on the kitchen island. It was the evening of September 6. His 27th birthday was three weeks away.

In a matter of hours, Mr. Jean would be dead. A white off-duty police officer who lived in Unit 1378 — directly below Mr. Jean — claimed that she mistakenly entered the wrong apartment after returning home from her 14-hour shift and believed Mr. Jean, who is black, was an intruder. Officer Amber R. Guyger, 30, fired her service weapon twice, striking him once in the torso.