He kept his eyes intently fixed on the Bohemian, and while the lively girl of sixteen danced and whirled to everyone else's delight, his reverie seemed to become more and more gloomy. At times a smile and a sigh would cross his lips, but the smile was by far the more pained of the two. (II.III.8)
You know that moment toward the end of the book when Frollo is staring at Esmeralda's corpse and he's compared to the spider with the fly? Think of this as something like foreshadowing. Esmeralda is young and lively, while Frollo is the opposite: prematurely old and gloomy. We get the sense that his gloom comes from the knowledge that he will never be able to get someone like Esmeralda. You could say that this is where all the evil starts.
"One fixed idea haunts me and pierces my brain like a red-hot iron." (VII.IV.23)
One aspect of Frollo's lust is his obsessiveness. It's something we've seen before in him, when it comes to his more-than-casual interest in alchemy. Frollo seems to be a guy who takes things to extremes—unlike, say, Phœbus, who doesn't choose to think too much.
He, who carried his heart in his hand, who observed no other law in the world but the good law of nature, who let his passions run off by his inclinations, and in whom the lake of powerful emotions was always dry, so assiduous was he every morning in making new channels to drain it—he did not know how furiously this sea of human passions ferments and boils when it is refused any outlet; how it swells, how it rises, how it overflows; how it heaves in inward convulsions till it has broken down its dikes and bursts its bed. (VII.IV.31)
Who says that Freud invented the idea of repression? This passage gives us some explanation of Frollo's dramatic behavior when it comes to lust. He might convince himself that he's not susceptible to lowly human wants and emotions, but according to Hugo, being in denial only makes things worse. It means that when Frollo does eventually have to confront his feelings, he's going to have no way of dealing with them.
His eyes glowed more and more, and everything betrayed the fact that Monsieur Phœbus was obviously reaching one of those states in which Jupiter himself did so many foolish things that Homer was obliged to call in the clouds for help. (VII.VIII.49)
The Roman god Jupiter was known to go to all sorts of crazy extremes in order to get down and dirty with mortal women. Think of it as temporary one-track mindedness. Phœbus, whose namesake just so happens to be that of another Roman god, is the same way: when he wants sex, that's all he can think about. It's a little bit different from the idea of obsession, but the point is that lust will compel Phœbus to do or say anything.
The swarthy, broad-shouldered priest, who had previously known only the austere virginity of the cloister, quivered and boiled at the scene. The sight of the beautiful girl in a tête-à-tête with the ardent officer seemed to infuse molten lead into his veins. He was extraordinarily disturbed. His eye plunged with jealous desire beneath all the undone buttons. (VII.VIII.50)
Along with obsession, another facet of Frollo's lust is jealousy. One of the reasons Frollo hates the idea of other people having sex with Esmeralda (remember how he makes Gringoire swear that he never did?) is because he himself has never experienced sex, being a priest and all. He's basically bitter that other people can do it, and he can't. To prove the point, you'll notice that his jealousy is mainly directed at "all the undone buttons." Sounds pretty sexy to us.
"In prison and on a bed of straw, a priest and a witch could discover delicious pleasures and melt into each other's arms." (VIII.IV.56)
Okay, Frollo basically has convinced himself that the only way he could ever "have" (nudge nudge) Esmeralda is to get her in prison. Does this make a whole lot of sense? No. Frollo himself admits that his plan didn't make a whole lot of sense. But in the grips of his desire for Esmeralda, it didn't matter. As he says, "When one is doing evil, it is madness to stop halfway." Like Phœbus when he gets into his Jupiter-like lusty state, Frollo's objective to sleep with Esmeralda is like a goal he has to pursue no matter what. We also learn from this passage that it really is sex, and not so much love, that Frollo is after.
"You think you are miserable. Alas! You do not know what misery is. It is to love a woman—to be a priest—to be hated—to love with all the force of your soul—to feel that you would give for the least of her smiles your blood, your life, your character, your salvation, immortality, and eternity, this world and the next—to regret that you are not a king, a genie, an emperor, an archangel, a god, so that you could cast a greater slave at her feet." (VIII.IV.62)
All right, you remember what we were saying earlier about Frollo being an all-or-nothing type of guy? Yeah, he doesn't seem to be able to go halfway when it comes to his feelings. One thing is for sure, though: he definitely thinks that between him and Esmeralda, he is the worse off. Pretty self-centered, right?
Make a note of that, because it tells us something about Frollo's character. He almost seems to see himself as a martyr. His declarations of unrequited love may be pathetic (in the sense of pathos), but it's hard to ignore that there is also something sinister and egotistical about them. This isn't the last time we'll hear his declarations of love, either; he seems pretty intent on giving voice to them.
Even in this extremity she saw him survey her nearly naked form with an eye glowing with lust, desire, and jealousy. (VIII.VI.82)
In the words of the great Tina Turner: "What's love got to do with it?" Esmeralda isn't fooled. She knows that if Frollo is obsessed with her, it's because he wants to get into her pants. The question is how much Frollo realizes this. The fact is, even though he still throws around the word "love" a lot, he seems to recognize that this love mostly has its basis in sex. Do you think that for him love and lust are the same thing?
Every night his frenzied imagination presented La Esmeralda to him in all those postures that had made the blood boil most vehemently in his veins. He saw her stretched on the wounded Captain, her eyes closed, her beautiful bosom covered with his blood, at the delicious moment when the Archdeacon had imprinted on her pale lips that kiss which the unfortunate girl, though half dead, had felt as a burn. (IX.V.8)
Despite all the blood and gore, there is definitely something sadistic and erotic about this image. But it makes sense that it would be the one that Frollo fantasizes about: Esmeralda is completely helpless, Phœbus is permanently out of the picture, and Frollo is in charge. So now we get a sense of what Frollo's feelings are really all about: he doesn't want to be mutually happy with Esmeralda; he wants to own her. See the next quote if you need more convincing.
"I will have you. You will not have me for a slave; you shall have me for a master. You shall be mine. I have a den to which I will drag you. You will come, you must come with me, or I will give you up! You must die, my beauty, or be mine—be the priest's, the apostate's, the murderer's! Do you understand? Come now! Kiss me, darling! The tomb or my bed!" (XI.I.64)
All right, now all those claims about love have been totally thrown to the wind. Think of this last moment between Esmeralda and Frollo as the moment when the truth gets boiled down. Frollo is all about power, control, and possession; and he's all about satisfying his own wants as opposed to anybody else's. This is where he finally becomes the quintessential evil character. Maybe before, we felt a degree of sympathy for him; after all, who hasn't experienced unrequited passion? But most of us don't set out to destroy the person who doesn't love us back. This is lust taken to its extreme.