The fatal shooting of an Arlington man was described in court yesterday as "a pure case of self-defense" by the attorney for former Pentagon guard Michael J. Gardner, on trial on a murder charge in the case.

Attorney Irving Starr said in his opening arguments to an Arlington Circuit Court jury that Gardner, 31, "feared for his life" May 10 when he shot 22-year-old Orlando C. Dominguez in Dominguez's apartment at 955 S. Columbus St.

Several residents of the apartment building testified they were startled the night of the incident by sounds of knocking on a door, shouts and a gunshot.

There was conflicting testimony from two children who were in the apartment at the time of the shooting.

Melido Garcia, 9, testified he heard banging on the apartment door and a loud voice threaten, "I'm going to kill you."

Tony Keastead, 12, who lived in the apartment with his mother and Dominguez, testified the man outside the door, whose voice he recognized as Gardner's, said only that he was a police officer and did not make any threats.

Keastead said Gardner's knocks at the door were not loud, and said Dominguez responded by saying, " 'I'm going to kill you. I'm going to cut your heart out.' "

In a taped conversation with police several hours after the shooting, played yesterday in court, Keastead said Dominguez took a knife from the kitchen "in defense of his own self."

Assistant prosecutor Arthur Karp maintained Gardner was not threatened and "knew what he was doing. He was thinking about what he was doing."

Gardner, who worked as a police officer with the General Services Administration, was arrested May 11 and released from Arlington County Jail a week later on $25,000 bond. The trial is scheduled to resume this morning.