PORTLAND — The murder trial of a Portland man began Wednesday with the question of whether the shooting was an act of revenge or self-defense.

Daudoit Butsitsi, 25, is charged with shooting Serge Mulongo, 24, six times on the night of Feb. 10, 2010. The two men had been close friends and roommates, but the relationship deteriorated before Mulongo was killed in the entryway of his apartment building at 218 Park Ave.

In opening statements in Cumberland County Superior Court, the prosecution and defense agreed that the case is not a matter of who killed Mulongo.

Assistant Attorney General Leane Zainea described Mulongo’s death as an ambush and execution. She said the two men had fought before, but always with their fists, until Butsitsi lay in wait for Mulongo and shot him with a .45-caliber handgun that night.

The state is not required to prove a motive to win a murder conviction, Zainea told the jury, but Butsitsi’s motive will be clear by the conclusion of the case.

“Plain and simple, folks. This is a case about revenge,” Zainea said without elaborating.

Anthony Sineni, Butsitsi’s court-appointed lawyer, told jurors that they must consider whether his client acted in self-defense.

Butsitsi — whose family calls him Kevin — knew Mulongo better than anyone else, Sineni said. He had seen Mulongo grow increasingly violent and unpredictable, and knew he had guns. And Mulongo had threatened to kill Butsitsi.

“He truly feared for his life,” Sineni said.

Sineni told jurors that they will hear about an unfamiliar echelon of society — people who live near the edge of society, who may not have the same education, opportunities and circumstances that they have.

He said “street smarts” will be a very important element of the case, as they came into play in the relationship between Butsitsi and Mulongo and their attitudes toward violence.

Sineni didn’t offer additional details about that aspect of the case, but it may have been a reference to the men’s shared history as immigrants who fled civil war in the Democratic Republic of Congo.

On Wednesday, the prosecution called its first five witnesses: neighbors who heard gunshots that night, two police officers who were at the scene, and the deputy medical examiner who did the autopsy on Mulongo’s body.

Marguerite DeWitt, who was the state’s deputy chief medical examiner at the time, described Mulongo’s gunshot wounds. She said three bullets entered his chest close together, maintained their proximity through his body and did not exit through his back — indicating that Mulongo may have been incapacitated and not moving at the time.

Tight clothing or the body’s presence against a surface can prevent bullets from exiting the body, she said.

“The body is telling me the body is not in motion at the time the wounds were received,” she said.