Suzanne Kreiter/Globe Staff

Court officers removed the man whose outburst interrupted the trial.

By Maria Cramer and Martin Finucane Globe Staff

The testimony of a key prosecution witness today in a 2010 quadruple slaying in Mattapan was interrupted when a spectator in the courtroom hissed that he was a “rat” and a “snitch.”

The man struggled as court officers removed him from the tense, crowded room and then yelled with an expletive that Kimani Washington, 36, was a “rat bastard.”

Washington, a career criminal, is under cross-examination today in Suffolk Superior Court. He is testifying in the trial of two other men charged with the slayings, his cousin, Edward Washington, 32, and Dwayne Moore, 34.

Washington, who has a string of assaults and drug arrests on his record, testified Tuesday that although he and Moore planned to rob a drug dealer of cash and cocaine, he left before the four people, who included a mother and her 2-year-old son, were killed.

Defense attorneys have argued that Washington, who has reached a plea deal with prosecutors under which he will serve 16 to 18 years in prison, is lying to save himself. They are trying to raise doubts about his credibility during the cross-examination, which stretched for hours today.

Washington acknowledged today that his lifestyle called on him to lie to police and to prosecutors.

The defense sought to show him in the act, playing a tape in which Washington lied to police investigators, telling them that a car that he had taken from the crime scene belonged to a friend when it actually belonged to one of the victims.