Read this about the principle of “Omerta”: https://en.wikipedia.org/wiki/Omert%C3%A0
Here is a story about the guy who almost certainly killed Bulger: https://usa.greekreporter.com/2018/11/02/the-greek-american-mafia-hitman-suspected-in-gangsters-murder/
According to the reports, the guy who killed Bulger is an incarcated Mafia hitman (who was not formally made a member of the mob because he and his Mafia brother were Greeks, not Italians). Bulger violated Omerta. The moment he was put into a prison population with other Mafia members his days became numbered. Word is that this stone killer also had a particular HATE for “snitches”.
He just demonstrated his loyalty and honor to the mob by disposing of Bulger.
At some time during the attack, Whitey Bulger's tongue was removed.
The guy that killed Bulger also killed the local mob boss in WMass. He also killed another mob guy and buried him in a shallow grave in the area.
He is a well connected bad guy. A very bad guy.