Spoiler warning: Spoilers for the complete series finale, Netflix Season 3.
Sully kills Dushane, then dies after Stefan chooses not to shoot him. Top Boy’s final season closes both men’s stories without revealing the identity of Sully’s killer.
Why does Sully shoot Dushane?
Dushane steals the drugs Jaq intends to return, hoping to finance an escape after the investigation into Jeffrey’s murder closes in. Sully catches him climbing a fence and shoots him.
Their last conversation takes place through that barrier as Dushane dies. The drugs remain valuable enough for Sully to recover, even after the partnership that built their position has ended in violence.
The fence makes their separation literal. They can speak about the rules they lived by but can no longer share a route out of the consequences.
Why does Stefan walk away?
He has a direct reason for revenge: Sully killed his brother Jamie. When he gets the opportunity to fire, he decides Sully is not worth it.
That refusal matters independently of what happens next. Stefan does not need Sully to live a happy life for his own choice to interrupt the exchange of one killing for another.
Treating the subsequent gunshot as proof the refusal was pointless would make Stefan responsible for controlling every other person Sully has harmed. The scene asks less of him, and something more personal: whether he will become the person holding the weapon when it fires.
Who actually kills Sully?
An attacker shoots him after he returns to his car. The figure is not clearly identified, and the ending provides no confirmed name.
Jaq and others can be discussed as theories because Sully has accumulated enemies. A plausible motive is not confirmation, and the series does not complete an investigation into the shooting.
The final uncertainty keeps the consequence larger than one revenge plot. Stefan can step away from Sully’s rules; Sully cannot simply step away from the world those rules created. His death ends the story without turning the audience into witnesses who know every answer.
Sources: official ending breakdown.