By Richard Valdmanis

BOSTON (Reuters) - Attorneys for accused mob boss James "Whitey" Bulger will seek on Wednesday to paint star witness John "The Executioner" Martorano as a heartless killer whose testimony cannot be trusted in the trial of one of Boston's most feared men.

Martorano, a former altar boy who has confessed to 20 murders, has spent two days on the stand for federal prosecutors describing chilling executions he said were either ordered or conducted by Bulger, head of the ruthless Winter Hill Gang during the 1970s and 1980s.

Free after serving 12 years for his murders - a sentence that was lighter because of his cooperation with federal authorities - Martorano is seen as a star witness whose testimony could form the backbone of the prosecution's case.

Under cross-examination by defense lawyers that began on Tuesday, Martorano resisted the label "hitman" or "serial killer," saying he "always tried to be a nice guy," never killed for money and believed murder was justified in defense of family and friends.

"My father always taught me that ... the priests and the nuns I grew up with taught me that," he told the jury.

In nearly the same breath he recounted several of his long list of murders, including one during which he stabbed a man to death following an altercation outside a Boston restaurant "because he wouldn't shut up."

Bulger, now 83, killed many rivals as he rose from small-time crook in a gritty Boston neighborhood to one of the most feared criminals in the city's history, prosecutors charge. He then disappeared and spent 16 years in hiding before his arrest in California in 2011.

His story has captivated the city for years and inspired the 2006 Academy Award-winning movie "The Departed."

He has pleaded not guilty to all charges of committing or ordering 19 murders.

Martorano told the jury that he, Bulger and their partner Stephen "The Rifleman" Flemmi regularly teamed up to murder rivals, sometimes accidentally gunning down bystanders.

"We were up to our necks in murders," he said.

Bulger played a variety of roles in their attacks, sometimes driving the car carrying the gunman or a support vehicle and sometimes helping to dispose of bodies. In one case, Martorano said, Bulger and Flemmi did the shooting - killing a bar owner as he spoke in a city telephone booth.

The gangster, who earned the nickname "Whitey" for the shock of light hair he sported in his youth, was quick to turn on associates he believed were talking too much about their criminal doings, Martorano said.

Bulger's trial stirred memories of a darker time in Boston's history - some of the killings described occurred just blocks from the waterfront federal courthouse that is the scene of what is expected to be a three- to four-month trial.

Bulger's willingness to kill associates he suspected of talking too much belied the fact that for years he traded information with a corrupt FBI agent, according to federal prosecutors. Bulger, through his attorneys, denies having been an informant.

But Martorano said the news that his former boss worked with law enforcement "broke my heart" and prompted him to break the gang's code of silence and testify, a deal that allowed him to secure a lesser sentence for his many killings.

(Editing by Lisa Shumaker)

[James "Whitey" Bulger holds John Martorano's youngest son, John Jr., during his Christening ceremony in this undated handout photo provided by the U.S. Attorney's Office in Massachusetts. REUTERS/U.S. Attorney's Office of Massachusetts/Handout via Reuters]