The character of Ambrosio (spoilers for film and book)

The book is a long Gothic novel that contains a deep and searing criticism of those in religious orders and, implictly, the way in which certain sections of society find themselves pressed into a religious order some times against their will.

Ambrosio of the book is much more evil than the film character. It's quite clear in the book that his rearing by the monks has accentuated the worst aspects of his personality and left him unprepared for any weaknesses or deficits he might have. Consequently he lacks compassion for others' deficits and weaknesses. The book Ambrosio exploits people and treats them as objects in the satisfaction of his desires. The film's Ambrosio was a more sympathetic character and his struggle and inner turmoil had a poignancy to it lacking in the book. Not that Ambrosio doesn't regret things or think otherwise about his actions in the book, but not enough to stop his acceleration to hell.

I'm not trying to break your heart,
I'm just trying hard not to fall apart