Friends, relatives and coworkers say 15-year-old Sean Smith was the kind of young man who called home on payday to see if his grandmother needed anything from the grocery store, a boy who worked a part-time job to buy the clothes that are so popular among teen-agers, an employe who did his job and was never late for work.

They say Smith was shot during an argument that started after someone stole his jacket, a $99.95 red ski jacket he bought at Athlete's Foot, where he was a sales clerk.

Smith, of 1231 I St. NE, became the 228th homicide this year in the District. Last year at this time, 197 homicides had occurred. In the past five months, six area youths have been killed, some by other youths. A 17-year-old youth was shot to death Dec. 11 after two teen-agers tried to steal his "boom box" radio.

"I don't understand so many teen-agers shooting and killing each other over such stupid things. I just don't understand why," said Oral Austin, the manager of the store where Smith worked.

Police had sketchy details on Smith's slaying. A spokesman said that at about 11:30 Monday night a man shot Smith and his cousin, Allen Mario Thompson, 18, with a handgun while the youths were arguing with other young people on a corner near their house. The youths were taken to D.C. General Hospital, where Smith died about 3 a.m.

Thompson, of the same address, was in critical but stable condition last night at D.C. General. Police were still investigating the case and had made no arrest.

Yesterday, several youngsters stopped by the row house where Smith, who turned 15 on Nov. 30, had lived for 12 years with his grandmother, an aunt, two brothers and seven cousins. They came to sit silently, to weep among friends and to ask why.

"It seems once you step off this block you're in the twilight zone," said Stephanie Brantley, Smith's 27-year-old aunt, who had helped raise him. "On this block there are elderly people, nice families. But there's a world out there where parents have no control over their kids, and it seems the streets are being flooded with guns.

"I have wondered when the police were going to do something about drugs around here," she said. "Now I wonder when they are going to do something about the drugs and the guns.

"These boys who got shot," Brantley said, referring to her nephews, "these were typical boys. They got into trouble that average suburban boys would get into. But I never had trouble with knives and guns."

Brantley said that on Sunday one of Smith's friends argued outside with a young girl and that the girl eventually went home and brought back a group of youths. Smith and one of the youths fought, and during that fight someone stole Smith's jacket, she said.

"He was a boy with pride," said Brantley, who works for Computer Data Systems in Rockville. "When he got that job, he came home and told his grandmother, 'I got me a part-time job. I'm not selling drugs to earn my money.' He took a lot of pride in that jacket. I guess a child doesn't think about the fact that they can buy another jacket. Sean just wanted his jacket back."

On Monday night, as Brantley came home from a spa, she noticed two groups of youths on opposite corners near the house. When her youths came in she questioned them about the incident and found out that they had been on one corner and that the group of youths with whom they had fought earlier had been on the other corner. Also, she learned that they had been arguing about the ski jacket.

"I told him, 'You can buy another jacket,' " she recalled. She went upstairs to her room and the youths played Monopoly in the living room until Smith got a telephone call.

"He must have gone back outside," Brantley said, tears welling in her eyes. "About 11:10, his oldest brother ran up screaming, 'Sean's been shot.' I went downstairs and saw Allen on the sofa. I lifted his shirt and saw a small hole in his chest. I called the ambulance and put something under his head, then asked where was Sean."

Brantley ran outside and found Smith, "a lump on the corner," she said. "I picked him up to carry him home. He said, 'I can't breathe. I hurt so bad. Please lay me down. Tell Momma {his grandmother} I've been shot.' "

She laid him down in the front yard in the cold drizzle. He told her he thought he was shot in his shoulder. She ran inside to get a pillow to place under his head. The police came, then the ambulances.

At the hospital, relatives, friends and a minister gathered in the emergency waiting room.

"The doctors couldn't stabilize him or prep him for the operation," Brantley said, tears streaming down her face. "I heard Code Blue announced two times. The doctor came and said, 'There's nothing else we can do.' I could hear the heart machine beeping. And then I heard it stop."

Brantley said Smith's cousins told her that a man with a gun had stepped out of the dark when they were on the corner and started shooting. "I don't understand why a man would kill two boys over a jacket," she said.

Yesterday, Smith's young friends gathered in the living room, decorated with poinsettias and tinsel, to talk about him and his love of basketball, fishing, rap songs and boxing. John (Ervin) Bradham, 16, one of his best friends, wept unabashedly as Brantley consoled him.

"He was one of the best friends I had," Bradham said. "We got into a little bit of trouble in school sometimes, playing around. But his heart was in the right place. He would even like people who didn't like him back. I never could understand that about him."

At the Athlete's Foot in Hechinger Mall, where Smith had worked a permanent part-time job as a sales clerk for three months, his coworkers also were trying to shake the shock of his death.

"He was a nice kid," said Austin, the store manager. "He did his job right. He was never late. He was fun to be with. He made you laugh all the time.

"Before I became manager, some of the managers used to pay him out of their pocket because he would come around and volunteer to take out the trash and clean the store," Austin said. "When I took over this store I called his house and asked him if he wanted a job. He came down, filled out an application and I hired him as a sales person. He ran stock, too."

Austin called in Smith to work Monday, and the youth worked until 9 p.m. Six hours later he was dead.

"I can't believe it," Austin said as he stood in the store, his voice choking. "He was talking and making us laugh. He begged to have {Tuesday night} off so he could go to the rap show at the Capital Centre. That was all he talked about Monday.

"He bought that jacket here. I just wish he had let it go," he said. "But I guess he didn't expect someone to shoot him over it."