That idea pretty much gets to the root of why Mordo works as a new villain: his point of view actually makes sense. It's hard to truly empathize with bad guys such as Kaecilius or Dormammu because their respective outlooks on the universe are utterly apocalyptic. By contrast, Mordo simply cares about rules, law and order. He abandons Stephen Strange and the sorcerers of Kamar-Taj not because he loses faith in his own mission or because he decides he wants to live a selfish existence; he strikes out on his own because he believes that Strange and Wong have lost their way. Mordo cares about upholding "Natural Law," and his views (despite their hard line, fundamentalist nature) make quite a bit of sense from an outsider's point of view. It's a perfect reversal on the typical hero/villain dynamic because Doctor Strange shows how the hero is the one willing to bend the rules for the greater good - rather than the other way around.