That's the question at the heart of the trial of George Zimmerman, a neighborhood watch volunteer who killed 17-year-old Trayvon Martin.
Monday marks the start of jury selection in Seminole County, Florida, where Martin was fatally shot on February 26, 2012.
The shooting put a national spotlight on Zimmerman's hometown of Sanford and sparked fresh debates about race relations and gun laws in America.
Zimmerman is Hispanic; Martin was African-American.
An initial decision not to pursue charges against Zimmerman led to the dismissal of the town's police chief and the appointment of a special prosecutor, who accused the neighborhood watch volunteer of unjustly profiling and killing Martin.
Zimmerman now faces a second-degree murder charge in Martin's death. He has pleaded not guilty and is currently free on $1 million bond.
WATCH LIVE VIDEO OF TRIAL:
"We don't need you to do that"
Prosecutors and defense attorneys agree on almost nothing about what happened that day.
What's clear so far is this: Martin left the home of his father's girlfriend in Sanford to get a snack at a nearby convenience store.
As he walked back, carrying a bag of Skittles and an Arizona iced tea, he and Zimmerman crossed paths.
Earlier, Zimmerman had called 911 to report a suspicious person in the neighborhood.
A recording of that call includes a police dispatcher asking the volunteer, "Are you following him?"
"Yeah," Zimmerman replied.
"OK, we don't need you to do that," the dispatcher said.
Zimmerman says he killed Martin, who was wearing a hoodie, in self-defense after the teen punched him and slammed his head on the sidewalk.
He suffered a fractured nose and cuts to the back of his head, according to a medical report by Zimmerman's family doctor.
Sanford police initially questioned Zimmerman and released him without charges. They said then there were no grounds to disprove his account that he'd acted to protect himself.
The case soon became the center of a national controversy, which continues some 16 months later, though at a lower intensity.
On Monday, protesters in at least six cities plan to demonstrate in support of Martin, who was unarmed when he was shot.
His family has said Zimmerman profiled the teen and crossed the line from neighborhood watch volunteer to vigilante.
Benjamin Crump, an attorney for Martin's family, has gone further, accusing Zimmerman of murdering Martin "in cold blood."
"In the fight of his life"
According to Crump, Martin was on the phone with his 16-year-old girlfriend shortly before the shooting.
The girl, who wishes to remain anonymous, says she heard someone ask Martin what he was doing and heard Martin ask why the person was following him.
She then got the impression there was an altercation, during which an earpiece fell out of Martin's ear and the connection went dead, Crump said.
Neighbors reported hearing gunfire.
Zimmerman recently waived his right to a pretrial hearing under Florida's "stand your ground" law, which allows people to use deadly force when threatened regardless of where they are.
His lawyers will claim self-defense. Zimmerman himself could testify at trial.
Defense attorney Mark O'Mara said he has no imminent plans to ask for a change of venue and would only do so if lawyers can't select a suitable jury.
"If we can pick a jury in Seminole County, this is where the incident occurred and this is where the case should be decided," O'Mara told HLN's Jean Casarez.
He also said the George Zimmerman defense fund has raised $85,000 in the past week and a half.
Media coverage of the case is expected to be intense.
The case garnered so much attention that about a month after the shooting, President Barack Obama spoke about it, saying the shooting required "soul-searching."
Zimmerman's brother, Robert, has called on the state to drop the charges.
"George lived in a community plagued by crime and was the first to come forward to help his neighbors," Robert Zimmerman said last month.
"George is a good, decent and honest man. It is now my honor to advocate for him. George is in the fight of his life quite literally."
Authorities initially "did their job when they refused to charge someone with a crime who committed no crime," he said.
"In this country, you don't charge someone why any crime solely to assuage the concerns of the misinformed masses."