What's with this "yandere, yandere" litany I keep hearing?
There isn't even the smallest hint of Nagisa being violent of brutal. There are no reasons to believe that she would resort to bloody violence if she's displeased or if she doesn't get her way.
As far as I can tell, with the data we have, my best guess is that she is a big fan of Chi-chan's first book... so big, in fact, that she wanted to meet her, become close to her, and (perhaps) have a amorous entanglement with her. Nothing yandere about this.