Here I was thinking the anime was taken a different path, like with
Akame ga Kill, but nope, Yandere in full force here! Not that I can blame her, the fuck is a maid doin' talking shit about a knight? Even if he is of a lesser birth, in terms of nobility, knights are still higher than a maid, which are typically enslaved or from a sub family. Which are maybe a step above commoners, maybe.
And even then, he's a knight chosen by the princess. That alone elevates him above even most nobility. So is this jealousy at play? Probably so. How did this commoner gain the princess's favor, is likely what is going through their minds. Well, it's not like it matters, in the grand scheme. A Yandere in power is a very scary thing.