Yes, there's clearly something dubious about assuming that not only Genghis Khan, but his entire army, consisted of weird mutants who somehow lack moral intuitions. Much more likely is that they had normal human moral intuitions, but failed to apply them generally to all people, rather than (say) people in their own cultural group.

Stalin actually was a psychopath (probably diagnosable, as he fits all the standard criteria: flat affect, deceives people easily and without remorse, indifferent to suffering, superficially charming). Genghis Khan may have been (we know far less about him). But the average Soviet soldier? The average Mongol warrior? Clearly not---there are simply too many of them for that to be plausible.