Regimes responsible for killing tens of millions are sadly not hard to find in the modern world either. Nazi Germany and Maoist China, for instance, caused the direct and indirect deaths of 17-20 million (per Business Insider) and 40-80 million people respectively (per the Washington Post). In the 1200s, these numbers meant more simply by virtue of there having been significantly fewer people. In regard to individual instances of Mongolian brutality, few rival their thoroughness in wiping conquered cities from existence.
Hitler and Mao took varying measures to conceal the extent of death under their rule, for the Mongols, it was known policy that even the animals within a rebellious city were unlikely to survive (though there were instances of certain people being spared and/or enslaved according to Exploring History). The siege of Baghdad is one of the more infamous examples. When the city refused to submit, the end result was thousands civilians murdered and/or mutilated, most buildings torn down, and the texts of the city discarded in the river (via The Great Courses Daily). The aforementioned sacking of Carthage is comparable, yet while that represented a decades-long buildup of aggression against a singular foe, for the Mongols, such destruction was nearly standard practice.