If familial mediterranean fever is untreated can it cause death?

Not directly: Fmf does not cause death directly (like say for example cancer or coronary artery disease). Patients with fmf do have higher rates of related diseases however, such as nephrotic syndrome or amyloidosis (also subfertility and miscarriages). These other diseases can all contribute to earlier death if the fmf is not treated regularly.

Very often it does: You would be very ill-advised not to accept treatment for familial mediterranean fever. Even if the episodes of serositis are not that bothersome, the risk in untreated fmf for developing amyloidosis is extremely high -- your health's ruined and soon enough your life's over. Colchicine slows / prevents this.