Diana is not a yandere, just a shonen protagonist who's getting deprotagonized and doesn't know how to deal with it.
that's such a sick idea, i love genre play like this.
although, wasn't she an otomege protagonist, not a shounen one?
In the original, the MC summons three dragon and uses them to battle another mage, fairly early on. I suspect it was at least a little bit more combat oriented then the typical romance. (It's actually described as an RPG app in the first chapter.)
With that said, Diana jumping to using cutlery to defend Natori struck me as a bit odd, particularly when she when she repeated the act with a fork hidden behind her back. It's not just the tendency towards violence that makes it odd, either. Cutlery just doesn't make for a good armament, especially when you have magic, and I'd have to imagine even an empty hand would be more useful than a fork, if she really thought she might need to defend Natori from the prince.
I wonder if her behavior was meant to foreshadow something about her past, or the state of the world, rather than her current mindset.