I half feel like it was the demons who tricked both families into attacking each other. It seems (based on MMCs reaction) that the red head girl is likely a demon too, so both family have one, and both are trying to get the best outcome for their chosen protagonist.
At least that's my best guess as to what's going on.
I'm also have suspicious that Eren is actually the original black-haired knight, and the demon's shenanigans caused them to switch places.
