A D.C. police sergeant, a chance customer in the store, shot and killed a robber during an abortive holdup in a Southeast convenience store Wednesday night in which the stickup man fired two shots that missed everybody.

Sgt. Arnold Nicholson, a 10-year veteran of the force, was off duty at the time. He was routinely put on administrative leave with pay pending an investigation of the 10:40 p.m. incident. The affray left Raymond Charles Thomas, 23, of 113 58th pl. NE, dead on the scene with "more than one gunshot wound to the torso," police officials said.

Police were still searching yesterday for Thomas' two unidentified companions, who fled after the shooting.

The shooting occurred at a 7-Eleven store at 3235 Pennsylvania Ave. SE, operated by H. Cha.

"There is much crime here, I have come to expect it," said Cha, 33, who has operated the store for 11 months. "But personally I feel terrible. I hate to have someone killed in my store. No matter what he did, he's a human and I fell sorry."

Cha's reaction typifies that of many Korean merchants in the District.

Koreans own about 50 percent of the "mom and pop" grocery stores in the city, according to real estate brokers, food wholesalers and city business leaders. Most are in inner city neighborhoods where cirme rates are high.

Lacking familiarity with the American language and customs, these businessmen have become easy prey for robberies, larcenies and shoplifing.

In December another Korean merchant, Boo Yeol Park, was fatally shot by a youth during a holdup at his Southwest Washington grocery store, police said.

Cha said an employe who asked not to be identified was standing behind the counter when Thomas and two other men came into the store Wednesday, picked up a few items from the candy and cupcake rack and walked toward the cash register.

When the employe rang up the first man's sale, "a 39-cent cupcake," Cha said, the second man grabbed for the cash in the drawer. "My helper then jammed the drawer shut on his fingers . . . [Thomas] pulled a gun and my helper jumped under the counter," Cha said.

Thomas then fired two wild rounds, investigators said.

Meanwhile, police said, Sgt. Nicholson, a member of the internal affairs division, was off duty, shopping in the store for a loaf of bread. Hearing the shots, he took cover, pulled his service revolver and identified himself to the three men as a police officier, police said.

When Thomas wheeled and turned his gun on Nicholson, police said, Nicholson fired at least twice, killing Thomas instantly. The other two men fled. No money was taken.

As Cha recounted the incident, Michael T. Lee, president of the 700-member Korean Businessman's Association, came by the store to reassure him. The network of Korean merchants in Washington works fast, he told a reporter. "We have to stick together."

"I'm thankful that for once an innocent man wasn't killed or harmed," Lee said. "Doing business in this area, it is a terrible situation. I never know what will happen.

"Some people say we should have guns in our stores and use them to let the people who would harm us know what we have, but I don't think that is the way. . . . We want to start a campaign to analyze the kinds of customers each store has so we can come to understand the neighborhood and its people. . . . With understanding, we can have better relationships with people, maybe less crime.

"So now they [Korean merchants] come and we tell them they can do well opening a small business -- a liquor store, carryout, 7-Eleven," Lee said. "More and more are doing it. But soon it will decline. Maybe the next generation, the kids now, will grow up and go into government jobs like other Americans do. They won't have these problems."