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.
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.