“He was just agitated about everything. Always shaken. Always agitated. Always mad.”

By the end of 2014, Mr. Mateen — the bodybuilder who had once imagined a respected future in law enforcement — was working the guard’s booth at the entrance to PGA Village, a golf resort community in Port St. Lucie. But even in this low-pressure position, he managed to unnerve and upset, especially when he seemed to think that he had been disrespected.

Heath Holtzclaw, who worked security at PGA Village a few years ago, has not forgotten how enraged Mr. Mateen became when he thought someone had given him an attitude. “You could tell he wanted to say something to whoever he felt had slighted him, but he never did,” Mr. Holtzclaw said. “He just slammed things around.”

Mr. Mateen would make people wait at the gate, sometimes causing delays, if he felt he had been disrespected, or if it was time for him to do his prayers.

Jasmine Kalenuik, a frequent visitor to PGA Village, came to dread encountering the guard at the gate — who, she said, “acted like a straight-up predator.”

“When I would go to grab my ID from his hand, he would cling to it and try to pull it back,” Ms. Kalenuik, 31, recalled. “He would hover over my car window and lean way in while breathing heavy on me with his teeth so clenched that you could see his jaw muscles sticking out.”