As has been pointed out, the ancient Greek
tyrannos really has nothing necessary to do with baby-eating flesh slurping fire-setting MWAHAHAHAs. It's really a
technical term describing a despot whose claim to power has not been legitimated in the traditional way. If you have countries ruled by some kind of parliament, hereditary rule, or some other kind of tradition, and Kyros comes in and just declares himself lord of everything, he's a tyrant. After all, for all the nobles who get shafted, that's already pretty 'evil'.
In this era tyrants were classified as bad specifically for their impact on the political order. He might be a wonderful guy, he might even be the most qualified man to rule your country, but you're just very worried about the precedent it sets. Hence Perikles was seen as a threat when he started being too smart for everybody else, and the Romans fetishised the legend of Cincinnatus, who 'tyrannises' for a few years then goes back to his farm.
The question is who's telling the story? A tyrant can be praised for bringing stability, order and law, while still being a genocidal murderer with veins of psychosis. Maybe the population of this setting celebrate mass killing of a neighbouring nation. If you look at the
diadochi, the successors of Alexander who broke up his empire, they were all basically tyrants - you then have Ptolemy establish a dynasty out of his tyranny that runs right down to Cleopatra, a bunch of Greeks that sit there and rule over the Egyptians.
I'm not sure what's the point of applying Enlightenment classical liberal criteria, really.
Now, as
Shadenuat says it would be cool to play someone who really is the right hand man of an evil psycho, but I imagine that would be difficult - it would actually be niche in a different way from other recent titles, in the sense that you're alienating people who like old school RPGs but also want to be the stereotypical chaotic good heroes. Hence you see things like Evil Genius go in the way of humorous bombastic evil, which I don't think is very interesting for an RPG.
You'd think this is perfectly made for some old Obsidian style deconstruction of classic morality tropes in the genre, though.