Let's face it, Frollo is hard to write.
I think Frollo is particularly hard to represent (esp. In fanfictios) if you have a very narrow, shallow and biased perspective of moral values of an European man from the Medieval times and Catholicism.
To think like a bad person is easy, to write a stereotypical villain is super easy, but I personally find Frollo (both Notre Dame de Paris and The Hunchback of Notre Dame versions) a character that goes beyond "this is good, this is bad", because he's not morally static.
Frollo's dichotomy is more clear in the book. He saved two defenseless children and raise them with actual paternal love, and he was never a troublesome person, in fact, he was super quiet and followed rules, yet he was also a misogynistic, a sorcerer (which is against the religious dogmas) and had zero trouble about almost kidnap and rape a teenager, plus get her killed. The thing with him is, he grew up with an hermetic view of life, he was raised by the punctual facts of books and religious dogmas, and lacked of human connections or lovingly feelings until he turned 19 and adopted Jehan.
He was emotionally neglected during the most important years of his personal development. In his case, to be religious was not a matter of actual devotion, but a mandate, something that came along with being an academic since in the Medieval times (and even now) to be educated in fields like philosophy is mandatory, and in order to be literate, the religious path was the "easiest" to commit. So I wouldn't say he was really attached to a spiritual crisis JUST because of his religious believes but HIS view of life and lack of understanding of healthy human relationships.
He didn't even had a relationship with his parents, how can we expect him to understand boundaries and the difference between actual romance and obsession? Especially during a historical context were this subjects were far to be discussed.
So imagine throwing a frozen sausage in boiling oil, that's Frollo when he got obsessed with Esmeralda.
And most of us can only imagine it, because now awareness exists.
So I think that to portray him as a mere religious man with a spiritual crisis is pretty short. Was he redeemable? Probably... But it would be hard, because even when he was juuussst about to understand the weight of his mistakes, he detached from it an still blamed others (Esmeralda). In the end, he didn't even feel guilty while seeing an innocent girl die because of him, he felt relieved. And that's very common to see but almost nobody admits it: it feels so good to see people you don't like suffering. It feels good to direct your frustrations to a different person and enjoy their misfortunes and justify it by saying they deserve it.
To write Frollo requires represent the balance of the good, and the hard pill to swallow of selfishness and victimization.
