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Boston (CNN) -- With defendant James "Whitey" Bulger sitting less than eight feet away and staring straight ahead, a star government witness in the case against Bulger explained Monday why he turned against the man he long counted among "my partners in crime, my best friends."

Aging mobster John Martorano said he learned Bulger was an FBI informant, the worst thing you could be in "Southie," a part of Boston that prided itself on absolute loyalty.

"It broke my heart. It broke all loyalties," said Martorano, his testimony laced with what sounded like sadness.

Bulger is charged in the deaths of 19 people during the nearly two decades that federal prosecutors say he was the head of the Irish mob in Boston.

Cullen: Entire FBI protected Bulger

2012: 'Whitey' Bulger: A life of crime

James "Whitey" Bulger rose to the top of the notorious Winter Hill gang, prosecutors say, before he went into hiding for more than 16 years after a crooked FBI agent told him in December 1994 he was about to be indicted on federal racketeering charges. He was captured in Santa Monica, California, two years ago, living under a false name with his girlfriend in an apartment in the oceanside city. Here are some other gangsters from America's past.

Al Capone, or "Scarface" as he was popularly known, remains one of America's most notorious gangsters. Known for wearing custom suits, fedoras and spats, Capone was infamous in 1920s Chicago for his bootlegging and racketeering activities. Capone died in 1947.

New York Mafia chief John Gotti was known as "Dapper Don" for his expensive suits and "Teflon Don" due to government charges failing to stick in three trials. He was later convicted of murder and racketeering. He died of cancer at age 61 in 2002 while serving a life sentence.

Henry Hill, a mobster-turned-informant for the FBI died June 12, 2012, at age 69. His story was the basis for Martin Scorsese's acclaimed 1990 film, "Goodfellas." Ray Liotta played Hill in the film.

George "Bugs" Moran was Al Capone's main rival in the Chicago mafia, culminating in the St. Valentine's Day Massacre in 1929 in which several members of Moran's gang were killed. Moran died in 1957.

Clyde Barrow and Bonnie Parker robbed banks across America before meeting their end when police and federal agents ambushed them on a dirt road in Louisiana in 1934.

John Dillinger, gangster and bank robber, was the first criminal to be called Public Enemy No. 1 by the FBI. Bureau agents gunned him down outside a movie theater in 1934.

George "Baby Face" Nelson, a car thief by age 14, associated with the likes of Al Capone and John Dillinger. Nelson died following a shootout with the FBI in 1934.

Mobster Louis Lepke Buchalter was one of the forces behind a hit squad known as Murder Inc. He died in the electric chair at New York's Sing Sing prison in 1944.

Vincent "Mad Dog" Coll was infamous for the attempted kidnapping of a rival gang member in 1931. Coll shot into a crowd, killing a child and injuring several other youths. Coll escaped conviction due to a lack of credible witnesses. He was later shot to death in 1932 while talking in a phone booth, most likely by a rival gangster.

Photos: American gangsters

Illustrations of Bulger and Greig were on display at a press conference in 2004.

Catherine Greig, longtime partner of accused mob boss and fugitive James "Whitey" Bulger, was sentenced to eight years in prison.

An FBI handout shows various images of Bulger, who became one of America's most wanted men after fleeing in 1995 before an impending indictment on racketeering charges.

Bulger sits with his onetime friend and confidant Kevin Weeks in Boston in 1994. Weeks later turned on his former boss. In 2000 he led police to the bodies of eight alleged Bulger victims buried around Boston.

Special agent Barry Mawn and U.S. Attorney General Donald Stern hold a press conference naming Whitey Bulger to the FBI's Most Wanted List in August, 1999. After more than 16 years on the run, Bulger and Greig were captured in California.

The couple had for several years hidden in plain sight in a three-story apartment building in Santa Monica, California.

FBI agents found assault rifles, a 12-gauge shotgun, many semi-automatic pistols and revolvers, a silencer, a Derringer pistol, multiple hunting knives and bundles of cash inside the couple's two-bedroom apartment.

Bulger, who is being held without bail, was the head of a South Boston Irish gang before he went on the lam in 1995.

Spectators and press crowd in front of the John Joseph Moakley courthouse in Boston as Bulger and Greig arrive for arraignment on June 24, 2011.

Bulger and Greig are shown during their arraignment in this courtroom sketch.

Whitey Bulger in hiding

For three decades, Martorano said, he was tight with Bulger and Bulger's partner, Steven Flemmi.

"They were my partners in crime, my best friends, my children's godfathers," said the 72-year-old divorced mobster who now lives on Social Security and who sold his life story in hopes of getting it made into a movie.

The men were so close Martorano even named his youngest son "James Steven" after his two friends.

In a flat, emotionless voice, former hitman Martorano began to describe some of the murders he said he committed, several of them, he testified, with Bulger along.

Martorano said he was the trigger man while Bulger was backup to ensure the "hit" was successful.

In one case, Bulger used his car to block a target coming out a side street. Martorano, who said he was firing out the back window from another car during the incident, testified Bulger "was afraid he was going to get shot because tracer bullets were coming out of the machine gun and flying over his head."

"Was he angry?" asked prosecutor Fred Wyshak. "No. He was joking about it," replied Martorano.

Martorano also described the killings of two brothers after one of them had angered Bulger's gang.

One brother was tracked down at a restaurant in Medford, Massachusetts, and "I shot him in the heart," Martorano said.

When the other brother was killed later, Bulger and his gang stashed the body in the trunk of a car then drove the car south and dropped it at a housing project in South Carolina where, as he had expected, "a few kids stole the car," Martorano said.

Under a plea deal, Martorano was sentenced to 14 years in prison in exchange for fully testifying against Bulger. He served 12 years and was released in 2007.

His testimony also was crucial against corrupt FBI agent John Connolly, who was found guilty in federal court on racketeering and obstruction-of-justice charges, among others.

Connolly served 10 years in federal prison and is now doing an additional 40 years in state prison after being found guilty for his role in the murder of Florida businessman and former World Jai Alai owner John Callahan.

Flemmi, Bulger's partner, is serving life terms without parole but avoided a possible death sentence by cooperating with federal authorities.

The connection between Bulger and Connolly also was the focus of Martorano's testimony Monday.

Martorano said Bulger grew up in the same Boston housing project as Connolly, who looked up to both Whitey Bulger and his brother William, who rose to serve as state Senate president in Massachusetts.

According to Martorano's testimony, when Connolly returned to Boston as an FBI agent, he thanked "Billy" Bulger for "keeping me honest" and told him "If there's anything I can do..."

Billy Bulger said, "If you could keep my brother out of trouble that would be a great help," according to Martorano.

Martorano testified that he, Whitey Bulger and the others agreed that Whitey would meet with Connolly and "be a good listener." It wasn't long before Whitey Bulger was giving Connolly money and buying gifts including unset diamonds for Connolly to give to his fiancee as an engagement present, Martorano said.

Prosecutors and law enforcement officials say it was the corrupt relationship between Whitey Bulger and Connolly that allowed Bulger's criminal enterprise to grow and even facilitated some murders.

The trial, which enters its fifth day Tuesday, is expected to take up to three months and has the potential to reveal sensational details about the mob and FBI corruption, especially if Bulger chooses to testify.

Bulger was in hiding for 16 years before he was captured in Santa Monica, California, two years ago, living under a false name with his girlfriend in an apartment.

At his July 2011 arraignment, he pleaded not guilty to the 19 murder charges and 13 other counts.