An Annapolis teen-ager was sentenced yesterday to a decade in prison for killing another boy who, he said, had robbed him at gunpoint four times.

Anne Arundel County Circuit Judge Pamela L. North told Dermot Chase Jr., 17, that she believed much of what he said about living in fear of threats and armed robberies by the 15-year-old victim, Timothy Lee Sembley Jr. But, the judge said, "It certainly doesn't make what you did right."

Chase admitted to shooting the victim shortly before 3 a.m. Sept. 9 in the College Creek Terrace housing project in Annapolis. He told authorities that Sembley had taken his money in the latest in a series of robberies and that he was afraid the robber was about to shoot him.

Chase said he had not reported the robberies to police because Sembley, who lived nearby, had bullied him since first grade and threatened to hurt him and his family if he told anyone. Instead, he got a gun.

"I can't fathom in a million years why you wouldn't have told anybody about this," North said, admonishing him for poor judgment.

After the hearing, Assistant State's Attorney Frank Ragione said, "I think it's a shame kids are out running around at 2:38 in the morning with handguns."

Sembley was a large youth with a lengthy juvenile record, said assistant public defender David Harding. The smaller Chase is an honor-roll student who bothered nobody, his relatives said.

Charged with first-degree murder, Chase pleaded guilty in February to manslaughter, a handgun violation and three counts of recklessly endangering another person. North sentenced him to 18 years in prison, suspended all but 10 years and ordered him placed on five years of supervised probation after his release. She asked corrections officials to consider him for a Patuxent Institution program for young offenders.

Martha Powell, Sembley's mother, said she had hoped for a longer sentence. She said Chase could have told her about any problems because her family has known the Chase family for years. She said she had no inkling that her son had a gun.

But Powell said she took comfort in Chase's apology to her in court.