(And before somebody brings up that TOS line about "no massacres under his rule"? Look what he did to the crew of Regula One. THEN tell me that Khan's not capable of massacres. He did it to them, he probably did it during his reign on Earth too.)
Why should anyone be asked to tell you that about Khan?
Capable? Sure, he's capable. Many people in such a position would be capable, even without being products of a breeding program or genetic engineering for superiority and world domination.
Massacre(s) committed
before he came to power? Possibly, though there's not a lot upon which to hang that supposition.
After his removal from power, yet
before he went into exile? Also possible, but as told, the story makes it seems as if the window for that opportunity would have been extremely narrow. (When one means to escape the planet undetected, along with several dozen other supermen who are in similar straits, committing an extra massacre wastes time and has a way of drawing unwanted attention.)
During his rule? At the risk of trampling on your certainty that Khan had to be so evil that he
must have carried out at least one massacre (possibly more) while he was in power, the statement in the episode is unambiguous:
Name, Khan Noonien Singh.
SPOCK: From 1992 through 1996, absolute ruler of more than a quarter of your world. From Asia through the Middle East.
MCCOY: The last of the tyrants to be overthrown.
SCOTT: I must confess, gentlemen. I've always held a sneaking admiration for this one.
KIRK: He was the best of the tyrants and the most dangerous. They were supermen, in a sense. Stronger, braver, certainly more ambitious, more daring.
SPOCK: Gentlemen, this romanticism about a ruthless dictator is
KIRK: Mister Spock, we humans have a streak of barbarism in us. Appalling, but there, nevertheless.
SCOTT: There were no massacres under his rule.
SPOCK: And as little freedom.
MCCOY: No wars until he was attacked.
SPOCK: Gentlemen.
KIRK: Mister Spock, you misunderstand us. We can be against him and admire him all at the same time.
SPOCK: Illogical.
KIRK: Totally.
No massacres. None.
I guess Khan is just not as eeeevil as you'd like him to be. (And yes, you
do tend to go on
ad nauseam about how sure you are concerning the absolute eeeevilness of Khan.)
Khan, a ruthless tyrant? That's beyond question, but when you take it upon yourself to invent acts for the character which are
in direct contradiction to what was established about him onscreen, the only thing anyone needs to
tell you about it is "No - that's your own invention. It's not really in the story. You can choose to believe whatever you want, no matter that the story says otherwise, but no one else is under any obligation whatsoever to accept or play along with what is essentially your fanfic version."