People are generally people wherever you go. There are some good, there are some bad. I imagine Tevinter is no more or less different than anywhere else.
Granted, the Tevinter people we meet are awful, but they're more or less all slavers. They're supposed to be asses. It's not like it's a country full of mustache-twirling villains.
Keep in mind that Tevinter took a lot of brunt from the Qunari invasion, and they never actually made peace with the Qunari. That means they've been fighting them for years, without the rest of Thedas (and, strangely enough, pretty much made Hawke a kind of folksie hero for killing the Arishok). Bad people are able to manipulate and put themselves into positions of power during wartime because they often have to 'do what is necessary'.
But let's not forget that Tevinter doesn't have a monopoly of really crappy political characters. Howe, for example, and he was going to be one of the top officials in Ferelden had Loghain succeeded. Then there's people like Meredith, the Lord Seeker, Gaspard, and so on. All insane, corrupt, and in positions of power that generally make people's lives hell.