Frollo is overrated. He believes he's in the right, yeah okay but the fact still stands he is still openly the bad guy. He acts, looks and dresses like one. Making him think he's good doesn't make him complex, it just makes him delusional. He's just a douche for the sake of it, someone to boo and hiss at.
The book version is superior; he actually does start off a decent guy who falls so far.
I wouldn't even call him the most realistic disney villain; Gaston is more realistic than Frollo; he gets what he wants, he never changes, he just improvises and plus it's not like he's even the villain to begin with. Gaston at first sight is sexist, arrogant and quite comedic in his arrogance but as the film progresses, he becomes more like a beast, more willing to achieve marrying Belle. He doesn't dress like a villain, he;s not well feared, he's the town hero with a ego the size of Mount Everest.
Frollo never goes through such change. He was willing to murder innocents before seeing Esmerelda and was willing to afterwards. Lusting over her doesn't change his character in anyway.