According to Merriam-Webster, "loyalty" is "a feeling of strong support for someone or something."
According to James "Whitey" Bulger, former South Boston crime king and leader of the Winter Hill Gang, and frankly, for anyone who grew up in Southie during his reign, the word has a hidden meaning. To be loyal means to never turn your back on your neighbors and especially to never "snitch" to the cops. If a person disregarded these rules, especially in relation to Bulger, they always paid for it, oftentimes with their lives.
The Southie brand of loyalty was ever-present in the newly released film, "Black Mass," where Johnny Depp does an impeccable job of portraying a cold and calculating Bulger. In the movie, it was clear that Bulger did most of his ruling by causing fear and panic in the city. His accomplices were compliant with him because if they crossed him they would have to face his extremely violent and sadistic wrath. The movie tends to focus on his relationship with FBI agent and eventually, his friend, John Connolly, played by Joel Edgerton. Connolly was also criticized for sympathizing with Bulger and neglecting to turn him in for a plethora of crimes that he was quite aware of. Of course part of the reason why Connolly didn't say anything at first was out of fear that Whitey might kill him, but as he mentions in the movie, the importance of loyalty to his Southie brethren is what really drives his uncanny behavior.
From the moment Connolly decided to ally with Bulger to catch an Italian mobster, he was consenting to manipulation. In the movie, Connolly appears to be rather lonely, having just moved back to Boston, and Bulger cleverly picks up on this vulnerability and uses it to his own benefit. As Connolly befriends Bulger, he recalls the Southie principles of loyalty, and he simply couldn't bring himself to snitch on his friend and confidant. While Connolly did abide by the definitions of loyalty, it also led him into a deep hole of corruption that he would not dig himself out of.
A major criticism of "Black Mass" is that it was inaccurate, and that Bulger was unethically glorified. According to one of Bulger's associates, Kevin Weeks, who was played by Jesse Plemons, the movie was "pure fantasy." However, it still teaches an important lesson about the dark side of loyalty. People always say to keep your friends close and your enemies closer, which is exactly how Whitey Bulger became so notoriously successful in his crime business. It is important to be faithful in friendship to an extent. But it is even more important to do the right thing, even if it feels wrong. Perhaps if John Connolly had considered this before he got involved with Bulger, he might not be spending 40 years in prison for conspiring with the notorious criminal, and wouldn't have been portrayed as an utter fool in "Black Mass."