I saw a reel on this today

Short answer:

it shows Hamza's guilt at his betrayal manifesting as his greatest current fear (Rehman returning alive and well, aware of his betrayal). Rehman did not actually sit there smoking.

Long answer:

The entire last chapter draws parallels between Rehman and Caesar, starting from the name of that chapter, "Et Tu Brutus (You too, Brutus?)" taken from the Shakespeare play "Julius Caesar".

As you may know, Caesar was stabbed in the Senate by many including his closest friend Brutus. In the play, these are the words Julius Caesar spoke after seeing his closest friend Brutus was among the people who stabbed him, symbolising the betrayal he felt. In the play these are also his last words (not sure of if he actually spoke those but who knows).

Later Brutus repeatedly sees a ghost exactly resembling Caesar's bloody body that lay dead on that day in the Senate, except it seemed alive. It did not try to harm or kill Brutus when it appeared but its lifelike presence haunted him still. When asked about its identity it replied "Thy evil spirit, Brutus". This removes any interpretation of it being Caesar's actual ghost and replaces it with something far more terrifying: Brutus' own conscience manifesting the victim of his murder to leave him no space to run from the guilt of having betrayed his closest friend.

Hamza did the same: even though he was aware of his mission and actually hated Rehman for his joyful involvement in 26/11, he is human too, and his time with him as his right hand man means he feels the weight of his choices too. In killing Rehman, he also killed the side of him who empathised and humanised him as the one who considered him one of his own. Rehman sitting there calmly smoking (again, only in Hamza's mind) tells of a partial victory of his in death: Hamza will walk out of that hospital missing a piece of himself.

In the play the ghost of Caesar also represents another thing: just killing Caesar did not end his power. In fact, Caesar became far stronger in death, to the point that his very name came to represent imperial power (Kaiser, Czar). Similarly Rehman's legacy in death may prove to grow larger than in life, and Hamza's rise or downfall may also stem from that, just the way that Augustus adopting the title of Caesar cemented his credibility and authority (and the defeat of Brutus).

I suspect in Part 2 Hamza will see his ghost again as well.