One of Mr. Mateen’s former co-workers said in an interview on Sunday that he had expressed concerns about the man’s demeanor when they both worked as security guards assigned to PGA Village, a resort in Port St. Lucie, Fla.

“He talked about killing people all the time,” the former co-worker, Daniel Gilroy, said.

According to Mr. Gilroy, who said that he had repeatedly complained to G4S, the security company that employed them, Mr. Mateen was a loud, profane presence who was prone to using racial, ethnic and sexual slurs.

Mr. Gilroy, a former police officer in Fort Pierce, described Mr. Mateen as a man who had “issues and just constant anger.”

“He was just agitated about everything, always shaken, always agitated, always mad,” said Mr. Gilroy, who said his relationship with Mr. Mateen became increasingly tense, with Mr. Mateen badgering him with text messages 20 or 30 times a day.

Mr. Gilroy, who joined G4S after a career with the Fort Pierce police and later left the security firm, said he could not provide names of any other co-workers who could support his account of Mr. Mateen’s behavior.

He expressed a measure of regret for not having pressed G4S to take more action.

“I kind of feel a little guilty that I didn’t fight harder,” Mr. Gilroy said. “If I didn’t walk away and I fought, then maybe 50 people would still be alive today.”

But he said he was not surprised to hear of what had happened in Orlando.

“I wasn’t shocked,” he said. “I saw it coming.”