But the point is, Khan was not written as a "madman" in either "Space Seed" or STID. The intent was not to portray him as some one-dimensional monster. He was clearly written as a nuanced villain with sympathetic motivations. Of the two main villains in the film, Khan was shown to warrant more of our sympathy than Marcus was. So I don't agree in the least that the film portrayed him as a "genocidal madman." Marcus was the one who was trying to start a war that could kill billions. Khan was trying to save his family, essentially, albeit willing to go to extreme measures to do so. Both were bad guys, to be sure, but Khan was the more sympathetic one.
I found nothing sympathetic about Khan. He was taking Enterprise senior officers and threatening to kill them if they didn't help him, he held a blade to McCoy's neck and didn't seem too sympathetic towards the security officers he pummeled in "Space Seed".
He also was going to have no issue killing all five-hundred people on the Enterprise if he didn't get his way.
As far as the "no massacres under his rule": what qualifies as a massacre? Plus, with the "Records of that period are fragmentary, however." comment, how are they certain that Khan wasn't some kind of homicidal monster? I'm certain a quarter of the Earth didn't just lay down their arms and decide it best to follow Khan.