Agreed, OP has seen too many movies. Other suspicious things (some of which suggest a longer-brewing confrontation):

- The "hint of a Russian accent" that Zac supposedly put on (but nobody except Shamji ever noticed)

- How quickly Shamji and his associates were to claim that Zac "seemed suicidal"

- The "heating up knives and clearing up blood" text from Shamji hours before Zac's death

- The traces of blood spotted by the police when they searched the apartment Zac jumped from

But hey, the cops asked a few questions and none of these shady characters instantly signed a confession so what more could they possibly do?