Because she is Pryde. She has the last boss cheat skills that made Pryde a challenging opponent. If you think about it, she's in a situation of being placed inside a game world as a specific, extremely important character and incidents keep occuring like they did in the game. How could she not be worried? She already noted in this chapter that she could have ordered the execution of a man without feeling anything too much about it. While she's not behaving now like the original, demonic Pryde, she feels like the circumstances will possibly push her into it, and it will be beyond her control.
At the end of the day, if a normal, especially young, civilian from a modern democracy is made the heir of an absolute monarchy, it should feel quite daunting. Absolute monarchies are a form of dictatorship. Think about how dictatorships work in RL. They always have human rights issues and corruption. It's inherent to totalitarian systems. Once she's the queen, she will hold the lives and deaths of the citizens in her hands, and she will, absolutely, need to put that power to use at times. That's the point where she should be the most afraid of finding out she's really Pryde.