
Black Mass is a biopic of the James "Whitey" Buldger who was known to ruled over South Boston under the Winter Hill Gang. The operations of the gang unexpectedly got a deal with the FBI to be a formidable asset. Though it would be beneficial on both parties, the fact that there is an illegal or criminal activity going on makes the alliance very complicated.
There is also the involvement of his brother played by Benedict Cumberbatch, who was a public servant by that time. Whitey also has a band of people who are either simple people to highly skilled killers which are forced to live with each other. It is really awkward and scary. This gave me the intensity of danger of the characters, no one is safe, and no one is innocent.