Ironically, it's a bowdlerized version of a similar scene in the book, where he's whipped and mocked by the crowd.
For some it is worse in this version because of how unprovoked that was. At least in the book he was guilty of attempted kidnapping and nobody knew that the true culprit was Frollo.
Judge Claude Frollo himself.
Most of the other Disney villains were terrifying and evil in their own right, but Frollo trumps a majority of them due to being very, very normal and realistic. It gets better. The chances of you personally getting attacked by a sea witch, a sorcerer, or an evil queen is slim to none. Are there genocidal racists out there who use religion to justify their beliefs and actions, and convince other people to follow them? Oh, yes.
His very introduction applies. As the Gypsies are surronded by soldiers, a huge shadow casts itself over the wall, revealed to be Frollo on his Bruiser of a horse. The pure terror in the male Gypsy's voice is as if he has come face to face with the devil itself; a textbook Oh Crap! reaction.
During the opening sequence, Frollo looks positively demonic as he chases down and ultimately kills Quasimodo's poor mother, a Gypsy he thinks has stolen something, and if not for the Archdeacon's intervention would very well have murdered baby Quasimodo himself as well. And what's worse is that Frollo thinks he is guiltless and in the right!
His obsession with Esmeralda is quite creepy indeed, and not at all subtle.
The scene where he confronts Esmeralda while she's about to be burned telling her to "Choose me or your pyre".
When he says "Let her taste the fires of hell," you can actually hear the faint screaming of a woman being burned alive! Chills.
In the italian version, he sings I'm waiting for you in Hell instead of just Choose me or your pyre. He knows this obsession'll damn him but it's got him too tight. That's a sign that he's really losing it, in a horrifying way.
Frollo looks very much like The Joker when he grins, and his cold glare is equally as frightening.
His final scene is the image above for a reason. While watching it, notice the color of his eyes and teeth. They're white like Esmeralda's and Quasi's, until he climbs the gargoyle to decapitate the Gypsy, then they turn yellow!
Imagine for a moment you are one of the soldiers under Frollo's command. Specifically, one of those attacking the Cathedral. You've stuck by the Judge through all of his monstrosities. The city is in revolt, the Cathedral is throwing beams, stone and molten lead at you. All of this is explainable. Then the birds attack and everything is put into perspective. The Cathedral isn't attacking you. God is and now the Devil has a claim on your soul. Pious or not, that would be terrifying.
The last twenty-five seconds of "Paris Burning" is the embodiment of fear and terror itself. In the film, it's played over a clip looking over the skyline of Paris, glowing bright red from all the fire and the smoke filling the sky, as though Frollo really has brought Hellfire to Earth.
The chorus of "Kyrie Eleison" (meaning "Lord, have mercy") make both nightmarish and heartbreaking as well.
And if you listen to the scene with headphones, as Frollo contemplates Esmeralda's escape, you can hear the muffled screams and cries of the townspeople - men, women, and children.
The scene where Frollo explains his viewpoint of the Romani people to Phoebus. Hello, blatant genocide metaphor in a kids' movie!
Frollo: I have been...taking care of the Gypsies. (crushes some ants with each word) One...by...one...
In the Finnish dub, Frollo's tone of voice when he says "This is an unholy demon. I'm sending it back to Hell where it belongs." is as if he's discussing the weather.
If you interpret the gargoyles strictly as part of Quasi's imagination, not to mention how he behaves around Frollo, it really puts into perspective how much psychological damage has been done to this poor boy in his 20 years in isolation.
Clopin giving a Slasher Smile when he was about to execute Quasimodo and Phoebus as spies. Thankfully Esmeralda stopped him before it was too late.
The burning of the miller's house just because he and his family were supposedly hiding gypsies. Said family included at least one child and a baby.
At the end of the stage version, when Quasimodo is about to throw Frollo to his death, Frollo cries out "You don't want to hurt me!" The gargoyles/chorus whisper to Quasimodo, "Yes you do."
It's taken Up to Eleven in the Paper Mills Playhouse version as you see Quasimodo's final grasp of sanity slip, ensures Frollo can't escape and hurls him off the roof.
Even worse is the Paper Mills version. In the German version, only one gargoyle says the line. In Paper Mills, it's the entire chorus. The effect is both chilling and awesome.
"Sanctuary (Reprise)" in the stage version. Taken Up to Eleven in the American production, in which Frollo corners Esmeralda in her cell and forces himself on her. The claustrophobia of the prison cell set as Esmeralda tries to crawl towards the bars and call for help despite knowing the cell is locked and nobody will free her just sells her helplessness and the discomfort of the scene.
Esmeralda: No - no! A demon! Help! Help, please!
As controversial as the sequel to the movie is, it does have the genuinely frightening Sarousch, whose treatment of Madeleine reeks of The Sociopath. Also chilling is the Adult Fear-riddled sequence where Sarousch holds Zephyr hostage to ensure his safe escape; Zephyr sounds legitimately terrified when his father (who he had previously bragged about and clearly idolizes) does nothing to help him despite Zephyr crying out to both him and Esmeralda and both parents are understandably beside themselves with worry.