An appeals court on Thursday threw out the murder conviction of a reputed gang associate in the fatal shooting of an off-duty Revere police officer nine years ago.

The unanimous decision of a three-judge panel of the Massachusetts Appeals Court potentially could lead to the retrial of Robert Iacoviello Jr., now 29, for the slaying of Officer Daniel Talbot, 30, in September 2007, behind Revere High School.

The court ruled that the trial judge erred when he failed to instruct jurors on the option to consider whether Iacoviello acted in self-defense or committed the lesser crime of manslaughter.

He was convicted in 2010 of second-degree murder, which carries a mandatory life term in prison, with the possibility of parole after 15 years.

Associate Justice Elspeth B. Cypher wrote that Talbot and other off-duty officers gathered behind the school after a night of drinking. Talbot spotted Iacoviello’s friend, another gang associate named Derek Lodie, and allegedly called him a “Blood Killer.’’

Lodie telephoned another friend, prompting a group that included Iacoviello to come to the school. Talbot approached Iacoviello’s group, and Iacoviello fired, striking Talbot in the head, she wrote.

Cypher said jurors should have been instructed on self-defense, since a reasonable juror could possibly infer “that Talbot pulled out his Glock and aimed it at the Iacoviello group, and possibly even fired it, before Iacoviello pulled out the nine-millimeter Luger.’’

A manslaughter instruction was also warranted to allow jurors to consider whether Iacoviello used excessive force in self-defense, she wrote.

Iacoviello’s lawyer, Sara A. Laroche, said her client is confident he will be acquitted at a retrial when “the jury is given the proper instructions.’’

Suffolk District Attorney Daniel F. Conley’s office said prosecutors expect to appeal the ruling, arguing that “a defendant who armed himself and went searching for a conflict was not acting in self-defense.’’

Iacoviello will remain in custody while the legal process continues.

Travis Andersen can be reached at travis.andersen@globe.com.