I'm skeptical of this.

    "To each [Mongol soldier] was allotted the execution of
    three or four hundred Persians. So many had been killed by
    nightfall that the mountains became hillocks, and the
    plain was soaked with the blood of the mighty."

  Some historians believe that over one million people died in
  the aftermath of the city's capture, including hundreds of
  thousands of refugees from elsewhere, making it one of the
  most bloody captures of a city in world history.

Really? Each soldier slaughtered hundreds of people within a matter of days or, at most, weeks? And the people never fled en masse, unlike the refugees who had fled before them?

  Excavations revealed drastic rebuilding of the city's
  fortifications in the aftermath, but the prosperity of the
  city had passed. 

Who would have been around to rebuild the city? I don't doubt the Mongols slaughtered large numbers of people or that they razed entire cities, but IMNSHO stories like the above are obviously myths.

If you look at the basic mechanics of well-documented atrocities at that scale--Rwandan Genocide, Holocaust, Khmer Rouge Killing Fields, Great Chinese Famine, etc--you'll see some combination of long time periods, very modern technology, and a sophisticated socio-political apparatus leveraged for population control. Without some mix of these things, you have no hope of killing enough people before they literally run for the hills and escape your reach.