> What they did to Cutie gave them no meaningful lesson whatsoever.

Did it need to? And arguably it did as they went through ordeals and these actions to try to succeed in their theory, which was a bad theory as you say. They jumped to conclusions and started to realize they were wrong and ignoring the constant of the book, the book telling them *exactly* how to get out and what they need to do. 

They just didn't want to acknowledge it as they were more focused on their mutual disdain. 

The whole time they're kind of escalating their attempts to *hurt* Rose, as their goal was to make her cry. This was kind of the culmination of that.