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.
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 Esmerelda is quite creepy indeed, and not at all subtle.
The scene where he confronts Esmerelda 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.
Frollo looks very like The Joker when he grins, and his cold glare is equally as frightening.
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.
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.