Former Boston Globe reporter Dick Lehr, who wrote a book about Bulger, described him as a coldblooded killer whose gang went to lengths to avoid detection.
"When they killed someone -- this is pre-DNA -- they pulled the teeth out, cut the fingers off, tried to make it so the victims, if they were discovered from their graves, couldn't be identified. There's just no bottom. It doesn't get much uglier than someone like Whitey Bulger," Lehr said.
Few people knew Bulger was a rat.
FBI agent John Connolly, who was raised in the same housing projects as Bulger, cut a deal with the alleged mob figure in 1975. Bulger would give information about the Italian mob -- the FBI's prime target -- authorities said.
Protected by the rogue FBI agent, Bulger got names of other informants who had dirt on him and rival gang members, people he is accused of killing.
He knew when police were watching, knew when they were moving in.
After he fled Boston, he spent more than a decade on the FBI's Top 10 Most Wanted Fugitives list before his 2011 capture.
His girlfriend, Catherine Greig, was sentenced to eight years in prison last summer for helping him evade capture.
Connolly is serving a 50-year sentence for second-degree murder and racketeering.
Prosecutors plan to call as many as 80 witnesses. Among them will be Connolly and Flemmi, who was also an informant for the FBI. He is serving life terms without parole but avoided a possible death sentence by cooperating in the hunt for Bulger.
On Wednesday, Carney called Flemmi "a psychopath without a conscience."
Other former Bulger associates are expected to be called by the prosecution.
Last August, Carney said his client planned to testify.
"At this point in his life, his goal is to have the truth come out regarding how he was able to act with impunity for so long in the city of Boston," Carney told CNN affiliate WCVB-TV.