Very certainly agree with you on Khan not being an irredeemable tyrant. Even in TOS they state that "unlike some other nations ruled by Augments, under his rule there were no massacres and no wars of aggression until he was attacked; he was thus among the most admired of the so-called "tyrants" into the 23rd century, being called the "best of the tyrants" by James T. Kirk. " - Memory Alpha

That says a lot right there. If even the victors noted in their history texts that he did not participate in the same atrocities as the others, then you know he couldn't have been truely irredeemable.