Sully killed Dris because Dris betrayed their gang, Summerhouse, by informing their enemy Jamie about the location of Sully and Dushane, which was a dangerous breach of trust in their violent and unforgiving world. Sully did not want to kill Dris, especially knowing Dris was a father to a young daughter, but killing snitches is part of the harsh "code" they live by. Sully shot Dris multiple times on a rooftop as a brutal warning and consequence for his betrayal, making it clear that crossing Summerhouse comes with deadly repercussions.
Dris’s death also signified the unforgiving nature of the drug game and its impact on family, as after his death, Dris's daughter Erin was placed in social services while her mother was in jail. Sully's act was driven by the need to enforce loyalty and stability in their gang amid ongoing conflicts and betrayals.
