@Vahneris The Princess said that she has a
separate mission that she has to do, and it seems like she can only do it while the other two are distracting the feudal lord and getting his magic seed. So by sabotaging their own mission, Rosie has actually jeopardized the Princess’ mission and put her into more danger.
Plus, his anger has nothing to do with not wanting her to sacrifice herself. You can read his internal monologue yourself: he’s angry because the Princess is putting her faith in Red. He hates that she just smiled at Red in a way that she’s never smiled for him before. That’s 100% jealousy and possessiveness, and he deserves all the vitriol he gets.