A Florida neighbourhood watchman shot dead an unarmed black teenager "because he wanted to", a prosecutor has said at the start of his murder trial.
George Zimmerman, 29, was a vigilante who presumed Trayvon Martin was up to no good, the jury of six women heard.
But a defence lawyer said Mr Zimmerman acted out of self-defence after Trayvon, 17, attacked him in Sanford, an Orlando suburb, in February 2012.
Mr Zimmerman has pleaded not guilty to second-degree murder.
The case drew national attention and has fuelled debate about race and the proliferation of guns in America.
Police waited 44 days to arrest Mr Zimmerman after the incident, in part because the volunteer watchman invoked a law called "stand your ground", which permits the use of deadly force if a person feels in mortal danger.
On Monday, the jury heard profanity-laced quotes from a phone call to police by Mr Zimmerman after he spotted the teenager walking in the gated community, in which the defendant tells an emergency dispatcher: "They always get away."
"George Zimmerman didn't shoot Trayvon Martin because he had to," prosecutor John Guy told the court.
"He shot him for the worst of all reasons: because he wanted to."
But a defence lawyer said Mr Zimmerman had been attacked first. "There are no monsters here," Don West told the jury.
He said that Trayvon used the street pavement like a weapon.
"Trayvon Martin armed himself with the concrete sidewalk and used it to smash George Zimmerman's head... That is a deadly weapon," Mr West said.
But prosecutors said none of Mr Zimmerman's DNA was found on Trayvon's body, nor was Trayvon's DNA found on Mr Zimmerman's gun or holster.
Trayvon's parents attended the first day of the trial but the teenager's mother, Sybrina Fulton, left the room as a recording of an emergency call moments before the fatal shot was played to the court.
The trial at a circuit court in Sanford is expected to focus on events leading up to the death.
Neither prosecutors nor defence lawyers dispute that Mr Zimmerman shot Trayvon in the chest with a 9mm handgun, nor that the teenager died at the scene.
Mr Zimmerman had been sitting in his vehicle observing the dark street when he saw Trayvon, who had his hood up while walking in the rain.
At some point the defendant, who had a permit to carry a concealed weapon, got out of his car and followed the teenager. Minutes later, the fatal confrontation ensued.
Mr Zimmerman has told police he lost sight of the teenager and was attacked as he walked back to his car.
But prosecutors say it was the defendant who started the argument.
There have been conflicting reports from witnesses about how the fight unfolded.
The trial is expected to last between two and four weeks, and the sequestered jurors will be constantly supervised by the local sheriff's office.