Sully shot Jamie because he saw Jamie as a major threat and rival in the drug business, particularly to himself and Dushane's plans. Sully had deep mistrust and suspicion of Jamie, knowing Jamie had previously attempted to have him killed and that Jamie had his own ambitions to take over as Top Boy. Despite Jamie proving loyalty to Dushane by killing his best friend Kit, Sully suspected Jamie was playing a long game to eventually betray them both. Sully was also emotionally volatile, driven by his own moral code and haunted by past traumas, which made him less willing to tolerate Jamie’s presence. The critical moment was when Dushane wanted to keep Jamie in the operation, but Sully’s and Dushane’s plans did not align. Sully ultimately took drastic action, seeing Jamie’s death as necessary to protect himself, his brotherhood with Dushane, and his position, shooting Jamie with multiple gunshots in front of Jamie’s younger brothers. This act also underscored Sully's decisive and violent nature in the world they operated in.