Why is this young man dead?

From Miami Herald:

Trayvon Martin traveled to Sanford on a trip with his father. The teen returned to his Miami home in a body bag.

Martin, 17, was shot and killed Feb. 26 by a neighborhood crime watch captain.

The circumstances of the shooting are still unknown, but the 26-year-old man who shot Martin, George Zimmerman, gave a statement to police that he shot in self-defense.

Martin’s grieving family said the teen went out around 7:30 on that Sunday during the NBA All-Star Game halftime to get snacks from a nearby 7-Eleven convenience store. He purchased Skittles candies and an Arizona iced tea for his stepbrother.

Somehow, on the way back to the Central Florida townhouse where Martin was staying, he ran into Zimmerman, who was armed with a 9mm handgun, reports said.

Martin was shot once in the chest.

“He had a gun, and Trayvon had Skittles,” said Benjamin Crump, the family’s attorney. “We want justice.”

I really don’t understand this. A neighborhood watch guy shot this young man? What?