The last time Talia Hyatt saw Dawnn Jaffier alive they were walking down Blue Hill Avenue to catch the end of J’ouvert, a predawn festival that kicks off Boston’s Caribbean Day parade. Both women had planned to wear flamboyant costumes — Jaffier’s was a jeweled number festooned with blue-and-yellow feathers — for the annual celebration of West Indian culture.

Then Hyatt heard gunshots. When she looked behind her, she saw Jaffier lying on the street, bleeding from the head. Hyatt ran to her, knelt by her side, and cradled her friend’s head in her arms.

“Stay with us. Stay with us,’’ she pleaded.

Hyatt, a 27-year-old accountant, wept as she recalled the scene on the witness stand Monday, the opening day of the murder trial of the two men accused of killing Jaffier, a 26-year-old youth worker. Jaffier’s slaying, on Aug. 23, 2014, shocked and angered those who knew her as a bright and caring mentor, daughter, and friend.

Prosecutors allege Jaffier was killed by two rivals, Keith Williams, 21, and Wesson Colas, 25, who were engaged in a testosterone-fueled beef to determine which one was the “alpha male’’ after they, along with their crews, exchanged icy stares in a convenience store on Blue Hill Avenue.

Assistant District Attorney Mark Lee said Williams fired the .357 Magnum that killed Jaffier and that, while Colas never fired his gun, he is legally responsible for Jaffier’s death because he drew first and instigated the gun battle that caused it.

Both men have been charged with first-degree murder and are being tried together in Suffolk Superior Court, where they sat just a few feet apart at the defense table.

“On that day, Mr. Colas and Mr. Williams were sworn enemies,’’ Lee said in his opening statement to the jury. “They entered into an agreement. They may not have known it at the time, but that agreement was, ‘We’re going to hurt each other.’ And that agreement ended up in the death of one young woman and an injury to another.’’ Lealah Fulton, who was sitting with her friends a few blocks away that day, was hit in the leg.

If Jaffier had been standing a few inches in another direction or had bent down to pick up a coin, she would be alive today, Lee said. He likened her slaying to the death of Milena del Valle, who was fatally crushed when the ceiling of a Big Dig tunnel collapsed on her car as she drove to the airport in July 2006.

Both deaths, Lee said, show “how cruel fate can be.’’

Lee said that although no one saw Williams fire his gun, gunpowder residue was found on Williams’s hand and shirt, and a woman watching from her third-floor window saw Williams running down the side street where his gun was later found, stashed beneath a porch.

John Galvin, Williams’s lawyer, said no witnesses saw Williams on Blue Hill Avenue on the day of the killing, even though Williams was wearing bright orange sneakers and a distinctive “Ninja Turtles’’ T-shirt. Galvin also said that only one particle of gunpowder residue was found on Williams’s shirt and hand, a minuscule amount that is considered a negative test result under FBI and State Police standards.

“The Commonwealth is making a stretch to tie Williams to this shooting,’’ Galvin told the jury.

Peter Marano, Colas’s attorney, said police never recovered the gun his client is alleged to have pulled because Colas didn’t have a gun. He said Colas was on his way to work when he stopped into the convenience store and that he didn’t have any beef with Williams.

“This man is not guilty,’’ Marano said. “This man, but for the grace of God, could be a victim.’’

On the witness stand, Ian Jaffier, Dawnn Jaffier’s father, spoke softly as he recalled the last time he saw his daughter.

It was the day before the Caribbean Day festival, he said, and he and his son, Ian Jr., were outside their home in Brighton, when Dawnn drove by in the van she used to shuttle children to and from the West End House, the youth-development agency where she worked. “She was always the busiest bee ever,’’ Ian Jaffier said.

Dawnn, he said, stopped the van to gently rib her brother, a college student with a basketball scholarship, about his broken arm.

“What are you gonna do now, baller?’’ she said. Then she had to get back to work. “I gave her a hug and my son gave her a hug,’’ Ian Jaffier Sr. said, “and the next day she was shot.’’

Michael Levenson can be reached at mlevenson@globe.com. Follow him on Twitter @mlevenson.