Devil's advocate here, posting after a long while since the way you guys are missing some facts and going off topic is starting to worry me
Am I the only one who actually remembered that the "ninja" and "sage" classes were exiled from their world and, as such, provide a reason as to why the princess couldn't do much against them? She was pretty much fighting against something largely unknown to her save for some stories of the past, not to mention the so called "fight" was as short as it could get, preventing the bitc-PRINCESS from doing further damage. Hell, it's even said verbatim on the very chapter the ninja-girl was revealed as such!
Also, for everybody bitching against the MC not trying to murder the bit... I mean, murder the princess; there are three reasons I can come up as to why he isn't trying to end her and, instead, trying to "save" her: first, as the first chapter shows, she had some kind of good relationship with the MC which, albeit short, provided some kind of attachment that prevented him from de-humanizing her the same way a soldier dehumanizes his enemies; second, the girl hasn't really tried to kill his family or close ones, just the hero so she can fulfill her mission, with the monsters not really counting for obvious reasons (the one time she tried to harm anybody other than the MC was to prevent the deus-ex-machina that was the dark priest class); thirdly and more importantly, the bi... screw it, the princess bitch isn't really evil per se, just tremendously misguided and with a wildly different set of morals and objectives than the rest of the "good guys", since she kinda needs to save her life, her kingdom and her entire freaking world as far as she's concerned, not to mention that there's probably an entire lifetime of indoctrination and (probable) brainwash involved.
Besides, let's be fair, if anybody here was in a similar position to her and had to choose between killing one guy or damning everything you've ever known and loved, you'd probably be acting pretty similarly to the bitch, though without the yandere tendencies and in a more effective way (the manga need to go on after all).