She seems to drive her opinions about Gen. Tilney from the books she has read

Catherine attempted no longer to hide from herself the nature of the feelings which, in spite of all his attentions, he had previously excited; and what had been terror and dislike before, was now absolute aversion. Yes, aversion! His cruelty to such a charming woman made him odious to her. She had often read of such characters, characters which Mr. Allen had been used to call unnatural and overdrawn; but here was proof positive of the contrary.

Then she's amazed that the general could sit in church before his wife's monument

...that he could sit so boldly collected within its view, maintain so elevated an air, look so fearlessly around, nay, that he should even enter the church, seemed wonderful to Catherine. Not, however, that many instances of beings equally hardened in guilt might not be produced. She could remember dozens who had persevered in every possible vice, going on from crime to crime, murdering whomsoever they chose, without any feeling of humanity or remorse; till a violent death or a religious retirement closed their black career. The erection of the monument itself could not in the smallest degree affect her doubts of Mrs. Tilney’s actual decease. Were she even to descend into the family vault where her ashes were supposed to slumber, were she to behold the coffin in which they were said to be enclosed — what could it avail in such a case? Catherine had read too much not to be perfectly aware of the ease with which a waxen figure might be introduced, and a supposititious funeral carried on. (Ch. 24)

The "dozens who had persevered in every possible vice" are not people that she could have known about in real life; obviously they were villains in novels.

But after Henry's "enlightment" she acknowledges what was her mistake:

Her thoughts being still chiefly fixed on what she had with such causeless terror felt and done, nothing could shortly be clearer than that it had been all a voluntary, self–created delusion, each trifling circumstance receiving importance from an imagination resolved on alarm, and everything forced to bend to one purpose by a mind which, before she entered the abbey, had been craving to be frightened. She remembered with what feelings she had prepared for a knowledge of Northanger. She saw that the infatuation had been created, the mischief settled, long before her quitting Bath, and it seemed as if the whole might be traced to the influence of that sort of reading which she had there indulged. (Ch. 25)