We understand, at this point in the novel, that Elegant Effendi’s killer is one of the miniaturists commissioned to work on the Sultan’s book. Pamuk writes chapters from the murderer’s perspective that acknowledge the perpetrator’s identity as both a painter and a killer. In these chapters, the narrator refuses to reveal his identity; this is a device by which the author can develop tension and create suspense. However, presumably, the reader has also encountered chapters written from the perspective of the killer under his workshop name, Stork or Butterfly or Olive. Two sides of the same man have been presented, with neither publicly admitting knowledge of the other.
This textual separation of identity, beyond being creatively interesting, indicates a more profound divide. In the chapters narrated by the murderer, the speaker insinuates that the act of killing has made him into a new man – it has stripped him of naivety, heightened his senses, and removed his fear of his own primal nature. Having taken a life, he finds his own to be immutably changed.
When the same man decides to extinguish Enishte’s life, the reader is once again privy to the killer’s division of identity. However, at this moment, it is not the killer ruminating on his own moral turpitude; the miniaturist’s depravity has become so pronounced that Enishte himself is cognizant of it. The narrator writes, as he looks up into his killer’s eyes, that “he was no longer the master miniaturist [he] knew, but an unfamiliar and ill-willed stranger” (173). This passage is almost self-referential insofar as it verbalizes a motif that emerged in earlier pages. Enishte’s epiphany at this moment is a poignant nod to the personal consequences of murder.
Your argument is really interesting, I also think the artist was inclined to commit murder because he was quite eager to finish the task which he thought would guarantee him an unprecedented reputation as a miniaturist and when he killed his brother, who happened to have been his rival since their early days as apprentices, he felt even more eager to become that skilled artist and more empowered in his tactics.