An undercover narcotics officer who was shot in the head Monday after making a drug buynin Southwest Washington died yesterday. Hours earlier, a 59-year-old man with a long history of involvement in narcotics was arrested and charged with murder, D.C. police said.

Office Bruce W. Wilson, 27, considered by law-enforcement officials as one of the top narcotics officers on the force, was pronounced dead at 1:07 p.m. yesterday at Greater Southeast Community Hospital. He had been shot once in the back of the head with what appeared to be a small-caliber handgun, police said.

Charged with murder was Avon C. Alexander, of 4347 Martin Luther King Ave. SW. Police said Alexander had been the subject of an intensive, month-long investigation into the distribution of a drug called Dilaudid. Used by doctors to treat terminally ill cancer patients, Dilaudid also is highly sought after by drug addicts who use it as a heroin substitute, police said.

Also arrested was Reginald P. Colbert, 39, of 913 26th St. NW, how, police said, sold Officer Wilson a drug called Preludin in pill form, which addicts use to heighten the effects of heroin. Colbert was charged with two drug law violations, but was nt involved in the shooting, police said.

The circumstances surrounding the shooting remained unclear yesterday.

According to police, however, two officers - Wilson and Peter L. Pressly - following up on a drug deal made last week, returned to teh 200 block of Elmira Street SW on Monday afternoon to make a purchase. Wilson made the buy, believed to be one pill of Preludin, which costs 40 cents with a legal prescription, but $8 on the streets, investigators said.

Pressly then arrested Colbert. While he took Colbert to a nearby car, Wilson decided to check the rear of the address for discarded pills, police said.

Police sources said Wilson apparently was ambushed while searching through a trash container. The sources said Alexander lives less than a block from where the shooting occurred.

Alexander's police record dates back to a robbery conviction in 1948. He has been convicted of a total of 13 felonies, and currently is facing drugviolation charges in Charles and Anne Arundel counties, Maryland, police said.

D.C. Superior Court Judge Leonard Braman set bail for Alexander yesterday at $50,000 Bail for Colbert was set at $1,000.

While members of the Metropolitan Police Department and Wilson's family mourned his death, Assistant Police Chief Burtell Jefferson pulled from his files an essay that Wilson had written the year he joined the force, entitled "Why I want to Be a Policeman."

"In the past several years, I've become greatly concerned with the crime situation," the young officer wrote. "Since the use of drugs has been a great reason for the upward climb in crime, the job of the policeman is not only to direct traffic but get involved with people and counsel them on such matters.

"The only thing about being a policeman than would concern me are the chances you have to take. But life is a chance, so that doesn't bother or even worry me. The most rewarding thing I hope to gain is security for my family, a comfortable income and a chance to work with people."

Wilson, (a member of the force for five years,) had received many commendations, including awards from the FBI and the U.S. Attorney's Office.