to the defense of our mc, he don't have much time or anything else, if he resets always to the day before the princess arrival, then he have a full day of work before him and even if he manages to get away, the same things that make him so unlikely as the perpetrator make him unable to do anything, he don't know anything about the royals, who for what reason would kill the princess, how exactly the princess was killed (even if it was poison, he still don't know where what how) and he has not really a possibility to find anything out, probably also no really close friends which he had to persuade every time anew, he can't even warn the princess properly if he don't know the murder details (disregarding not being deemed crazy)
but this is really not what I was expecting, thought he would be an isekaied librarian who gets the heritage/spirit of someone and sets forth to fulfill his whatever heroic fate