From his perspective, he's saving Paris from pagan customs. Something kind of heroic, to some extent he shows care towards others. From everyone else's perspective, he's out to ruin people's lives. Something strictly villanous.
Now when he wants to get it on with a girl, a pagan one, his one "heroic" trait falls into question. Its big deal for him because lust is a cardinal sin. Then as he burns all of Paris he commits another big sin, wrath, without even identifying it.
Now since the beginning of the movie you see he never really followed God's rules. He does show wrath, killing quasimodo's mother... but... she kind of had it coming.
Wrath happens when strong emotions make you act outside of reason, and quasi's parents behavior, seeking to operate in the shadows, made them criminals. Moreso when the mother resisted arrest. Reason told that they had to be prosecuted... but not killed. He did show wrath killing her, but in the end, as sad as it is, Paris wasn't going to care. Frollo could unleash wrath, but in a way that didn't damage his public image.
Now when Frollo prosecuted Esmeralda, reason certainly didn't tell that he could burn an enitre city to find one person. If his mission was to save Paris from pagan customs, that decision did more damage to Paris than it made pagan customs banish from people's minds. Paris cares now.
So... Frollo was a real villain from beginning to end, but the question is, to whom. In the beginning he was a real villain to the cardinal, only witness of his henious crime (along with a bunch of statues)... but not with enough power to trump a public official. But when Frollo met with a moral dilemma, he lost his mind, and got worse at hiding his most natural reactions from the world.