Zurab has prominent cauliflower ears, a condition that has been linked with hearing loss and tinnitus (via Healthline). The cauliflower ear is a result of physical trauma, and the movie heavily implies that his own father is responsible for this, since we see him badly beating Zurab during flashback sequences. As such, the hearing loss gives the villain a stealthy dose of tragic backstory — and somewhat explains his stoic nature, given that his impaired hearing means he doesn't necessarily always have an easy time communicating with the outside world.
With his hearing aid, Zurab can clearly operate his day-to-day life well enough to, say, use the phone, but when we see him remove the aid from his ear, he immediately starts hearing a mysterious, eerie chattering. Sergo (George Lasha) reveals that people are aware of this, and think that Zurab can communicate with biblical entities. Fantastic as this part of the plot may seem, it turns out that tinnitus — an ear condition that cauliflower ears can potentially cause, remember — can manifest as auditory hallucinations (via the National Library of Medicine).
Say what you want about Zurab's increasingly bloodshot eye, facial scars, and tattoos, it seems that the movie has done its homework regarding his hearing loss. Though there's no denying that Zurab is an evil and violent crime lord, this gives him subtle hints of tragic villainy and further explains why he's so willing to put his own life on the line to accomplish his mission to kill Tyler: Not only does Tyler kill the person Zurab has ever sworn to protect, but the gangster apparently also has to deal with constant, tormenting auditory hallucinations ... courtesy of his own father, no less.
