Well then you're doing a disservice to Tolkien.
Tevinter is no less evil than the rest of the countries in the DA universe. All governments do whatever they can to maintain power. The only difference with Tevinter is that they're all mages, and all mages are bad k? Despite what we may be told, slavery isn't vilified by everyone either, and we still don't know how said slaves are treated by the vast majority of their owners. Slavery with bad treatment is unacceptable, but slavery with decent-good treatment is questionable. I'm not even going to touch the sexuality/gender part.
Spare me Dermain, Tolkien inspired is a common catch all term used to describe a series that uses the common tropes found in most medieval fantasy universes, while it may not be Tolkien per se that inspired these fantasy universes but rather Dungeons and Dragons (which was inspired by Tolkien) you can't deny that the popularity of using many of the common fantasy tropes that many use in the genre did stem from Tolkien, what would you prefer me to call the genre?
But back on the subject, you still haven't listed Tevinter's redeemable qualities that make them seem like less of a nation of total assholes, and while we don't know how EVERY slave owner treats their slaves (though even if the owners aren't outright dicks to their slaves slavery is still a dick move) we have seen several glimpses of what they can expect from being used to fuel blood magic (which is the cause of every problem in Thedas because reasons) rituals to being subject to experiments where they have lyrium grafted to their skin, not sure the existence of a slave in Tevinter is a pleasurable one.
While the other nations of Thedas have their elements of douchebaggery they are generally nowhere near as big a douchebags as Tevinter and dare I say even show a level of idealism and progressiveness (especially if they find themselves under the guidance of the noble king Alistair) that would seem out of place if we were to take Dragon Age serious as a Dark Fantasy universe like the developers so often tried to market it as.
Also while I never said "Mages are bad k" that does seem to be a common theme in Dragon Age 2 so perhaps that is another trait the writers intended to clue the player in that Tevinter are the bad guys? (don't take that last line seriously, while I would hope I would not need to point that out the last part of your post where you lecture me on the nature of schizophrenia and why Loghain does not have it seems to suggest otherwise)
Why does he only have to be one of those two (I'm ignoring the villain part for the moment)? Loghain can be BOTH a misguided patriot and a power hungry usurper, it's not really a leap. If Loghain believes that Cailan's policies will endanger the stability of Fereleden and he also believes that he's the only one that can save it, then it's not contradictory. It's only contradictory if you can't see a person as well being a person by consisting of good and bad traits.
Is Loghain doing it because he loves his country and wishes to protect it at any cost or is he doing it because he desires power and prestige and doesn't care what he has to do in the process even if it involves betraying his King and destroying the very county he supposedly loves? These are 2 contradictory motives, it is entirely possible for a person to consist of both good and bad traits, in fact this is one of the qualities of being human and I challenge you to find anyone who consists of either only good or only bad traits but the traits that Loghain show contradict each other as if there were 2 entirely different people inhabiting the same body.
As for being a mustachioed twirling villain, he's not. If he believes he's the only one that can save Fereleden then he is understandably going to try to stop people that he believes would hinder that. He has other motivations besides "AHAHAHAHAHA I AM EVIL! I DO EVIL THINGS! AHAHAHAHA! DIE, DIE, DIE, DIE, DIE! AHAHAHAHA!" like actual mustachioed twirling villains...
Well the evidence certainly does paint him as such, I mean I find it hard to believe that even the power hungry usurper would do what Loghain does as some of his actions serve no real purpose other than to establish what a dick he is and provide evidence for the player to use to turn people against him. I mean he does not even try to explain the whole selling the elves into slavery thing, his reason for selling the elves into slavery is that the Alienage is a squalid hellhole so his solution was to sell the elves into an even worse existence? What!?!? Not sure what he hoped to gain by poisoning Eamon (which he had already set in motion before Cailan was abandoned) other than keeping a large portion of the Ferelden military out of action in a time of crisis.
Then you have the part after the battle plan at Ostagar where he turns and gives the camera a sinister look and says "yes Cailan, a glorious moment for us all" in a tone that implies that he is about to do something nefarious, the only thing missing was the sinister laugh with lightning flashing in the background.