that Ghengis Khan won because he was actually a moral person
He was probably as moral as any war-leader who conquered an empire. As you said of other people involved in wars, some believe very strongly that their cause is just, even if later generations judge otherwise.
The moral worth of a leader can only be understood in terms of what he himself believes to be right conduct: whether he adheres to the demand of his culture's mores as a person, and whether he serves his people faithfully as a leader. A man who sabotaged the collective aspirations of his nation would be judged as an unethical leader [dishonest, disloyal, perhaps even treacherous] even if he did it in order to save a million lives, and though posterity might see him as virtuous. A man who takes upon himself the fulfillment of his people's aspirations, even though it entails duplicity and ruthlessness and results in a million deaths, is seen a principled leader, and his personal vices are overlooked, maybe even celebrated --- so long as he wins.
In either case, the degree of private evil he indulges has no bearing on his reputation as a leader.
The amount of evil he manifests in public matters only so far as it affects national morale. (He can torture all the prisoners of war he likes, so long as he shows appreciation to his generals and provides bread and circuses for the people. )
The reason we have a word and concept for evil is that we all carry it in us. We can identify it because we recognize it as very real and sometimes compelling part of our own nature. It can't be eradicated.
However, each one of us, in order to be successful social animals, must learn to keep it under control.
And we, quite rightly, condemn those who indulge the evil part of nature, while we tend to forgive those who try to control it an fail.