I can see how any reader would be disappointed. Catherine seems to have been making some progess/on the right track in learning to 'read' people, especially due to her association with Elinor and Henry,(Henry's guidance being a major factor), so that it seems a regression when she goes off on a tangent concerning the general. There are clues to help prevent Catherine from taking the leap of imagination to the level she does. At the same time, I think JA does a sufficient job in setting Catherine up to make this very leap. The choice was hers, I think. She could have made use of her newfound ability to see things as she should have or go with her great urge to immerse herself in the Gothic experience. The choice she makes, I feel, is based on several things as provided by JA: her overwhelming desire to experience something horrid, Henry's teasing tale on the way to the Abbey, and then her incredible disappointment in the Abbey once she arrives. She seems on a determined quest to get her experience of the horrid even if she has to practically make up something on the slimmest of evidence.

The thing is, we discover in the end, that while her imaginings of mayhem and murder were way off base, her instincts about the General being an unjust and cruel man were not so wrong. From the very beginning Catherine wondered about the relationship between the General and his children. They appeared almost robotic in their response to him, just going along with the program in my opinion, and this is what I think Catherine observed. It is probably something the Tilneys developed in their childhood, no doubt their mother too, to keep the peace. I don't think they ever bothered to delve too deep into their father's character other than to notice his penchant for money and things and punctuality in others. Catherine noticed the immediate change in atmosphere even when the General left the room, not to mention when he left the Abbey.